aarch64/fenv: fix fenv_t

[inkydragon]

AArch64 has two 64bit regs, AArch32 only has one.
The high 32 bits of both registers are reserved, so we only need to save the low 32 bits and pack them into a uint64.

• copy aarch64/fenv.h from FreeBSD
  https://github.com/freebsd/freebsd-src/blob/de1aa3dab23c06fec962a14da3e7b4755c5880cf/lib/msun/aarch64/fenv.h

Fix #278

[inkydragon]

Tracked by #320

Do we want to sync openlibm_fenv_arm.h from FreeBSD too?

Seems there are some bugs:

openlibm/include/openlibm_fenv_arm.h

Lines 65 to 67 in 5a88ede

	/* We need to be able to map status flag positions to mask flag positions */
	#define _FPUSW_SHIFT 16
	#define _ENABLE_MASK (FE_ALL_EXCEPT << _FPUSW_SHIFT)

/* We need to be able to map status flag positions to mask flag positions */
#ifndef __ARM_PCS_VFP
#define	_FPUSW_SHIFT	16
#define	_ENABLE_MASK	(FE_ALL_EXCEPT << _FPUSW_SHIFT)
#endif

And FreeBSD doesn't use _ENABLE_MASK
https://github.com/freebsd/freebsd-src/blob/de1aa3dab23c06fec962a14da3e7b4755c5880cf/lib/msun/arm/fenv.h#L83-L87

#else	/* __ARM_PCS_VFP */

...

#define _FPU_MASK_SHIFT	8

https://github.com/freebsd/freebsd-src/blob/de1aa3dab23c06fec962a14da3e7b4755c5880cf/lib/msun/arm/fenv.h#L113

[ViralBShah]

Please merge if ready.

[inkydragon]

All CI passed. Assume everything works well.

[lazka]

This fails to build on Windows, any ideas?

clang  -O2 -pipe -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong -Wp,-D__USE_MINGW_ANSI_STDIO=1 -fno-builtin -fno-strict-aliasing  -std=c99 -Wall -I/c/M/B/src/build-CLANGARM64 -I/c/M/B/src/build-CLANGARM64/include -I/c/M/B/src/build-CLANGARM64/aarch64 -I/c/M/B/src/build-CLANGARM64/src -DASSEMBLER -D__BSD_VISIBLE -Wno-implicit-function-declaration -I/c/M/B/src/build-CLANGARM64/ld128 -c src/s_fma.c -o src/s_fma.c.o
In file included from src/s_fma.c:31:
In file included from C:/M/B/src/build-CLANGARM64/include/openlibm_fenv.h:6:
C:/M/B/src/build-CLANGARM64/include/openlibm_fenv_aarch64.h:39:9: error: unknown type name '__uint64_t'; did you mean 'uint64_t'?
   39 | typedef __uint64_t      fenv_t;
      |         ^~~~~~~~~~
      |         uint64_t

[jeremyd2019]

it looks like other arches' headers (openlibm_fenv_{arm,amd64}.h at least) use the standard non-double-underscore names for these types.

[lazka]

yeah, that seems to have done the trick: https://github.com/msys2/MINGW-packages/blob/356bd84ad0398d2e063448ad7689fa5f04d7a00c/mingw-w64-openlibm/0002-fix-aarch64-build.patch