Fixed linkage in remote_bitbang library

[TobiasFaller]

This pull request fixes the linking of the bitbang library under modern GCC version where the multiple definitions of the global state results in the following error:

#> gcc (Gentoo Hardened 10.3.0 p1) 10.3.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

#> make all
cc -MT remote_bitbang.o -MMD -MP -MF ./.d/remote_bitbang.Td -std=gnu11 -fno-strict-aliasing -Wall -Wextra -Wno-missing-field-initializers -Wno-unused-function -Wno-missing-braces -O2 -g -march=native -DENABLE_LOGGING -DNDEBUG -fPIC -I./  \
        -c  remote_bitbang.c -o remote_bitbang.o                                                                                                                                              cc -MT sim_jtag.o -MMD -MP -MF ./.d/sim_jtag.Td -std=gnu11 -fno-strict-aliasing -Wall -Wextra -Wno-missing-field-initializers -Wno-unused-function -Wno-missing-braces -O2 -g -march=native -DENABLE_LOGGING -DNDEBUG -fPIC -I./  \
        -c  sim_jtag.c -o sim_jtag.o
ld -shared -E --exclude-libs ALL -o librbs.so  \
        remote_bitbang.o sim_jtag.o
ld: sim_jtag.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:25: multiple definition of `recv_end'; remote_bitbang.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:25: first defined here
ld: sim_jtag.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:25: multiple definition of `recv_start'; remote_bitbang.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:25: first defined here
ld: sim_jtag.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:24: multiple definition of `recv_buf'; remote_bitbang.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:24: first defined here
ld: sim_jtag.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:21: multiple definition of `client_fd'; remote_bitbang.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:21: first defined here
ld: sim_jtag.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:20: multiple definition of `socket_fd'; remote_bitbang.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:20: first defined here
ld: sim_jtag.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:18: multiple definition of `quit'; remote_bitbang.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:18: first defined here
ld: sim_jtag.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:17: multiple definition of `tdo'; remote_bitbang.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:17: first defined here
ld: sim_jtag.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:16: multiple definition of `trstn'; remote_bitbang.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:16: first defined here
ld: sim_jtag.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:15: multiple definition of `tdi'; remote_bitbang.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:15: first defined here
ld: sim_jtag.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:14: multiple definition of `tms'; remote_bitbang.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:14: first defined here
ld: sim_jtag.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:13: multiple definition of `tck'; remote_bitbang.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:13: first defined here
ld: sim_jtag.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:11: multiple definition of `rbs_err'; remote_bitbang.o:/root/uni/various/pulpissimo/pulpissimo/rtl/tb/remote_bitbang/remote_bitbang.h:11: first defined here
make: *** [Makefile:87: librbs.so] Error 1

With GCC 9.3.0 the linker ignores the multiple definitions:

#> gcc-9.3.0 --version
gcc-9.3.0 (Gentoo Hardened 9.3.0-r2 p4) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

#> CC=gcc-9.3.0 make all
gcc-9.3.0 -MT remote_bitbang.o -MMD -MP -MF ./.d/remote_bitbang.Td -std=gnu11 -fno-strict-aliasing -Wall -Wextra -Wno-missing-field-initializers -Wno-unused-function -Wno-missing-braces -O2 -g -march=native -DENABLE_LOGGING -DNDEBUG -fPIC -I./  \
        -c  remote_bitbang.c -o remote_bitbang.o
gcc-9.3.0 -MT sim_jtag.o -MMD -MP -MF ./.d/sim_jtag.Td -std=gnu11 -fno-strict-aliasing -Wall -Wextra -Wno-missing-field-initializers -Wno-unused-function -Wno-missing-braces -O2 -g -march=native -DENABLE_LOGGING -DNDEBUG -fPIC -I./  \
        -c  sim_jtag.c -o sim_jtag.o
ld -shared -E --exclude-libs ALL -o librbs.so  \
        remote_bitbang.o sim_jtag.o

[bluewww]

Thanks for the PR. Sending this through our CI

[bluewww]

lgtm thanks