Consolidate codegen-related compiler flags

[alexcrichton]

We're starting to grow a very large number of codegen-related compiler flags, and arguably these should all be documented in one codegen place rather than --help.

Things I believe need to be consolidated:

• no-pic (Add no-pic debug flag to disable position independent code #11995)
• target-feature
• target-cpu
• no-rpath
• ar
• linker
• llvm-args
• -Z asm-comments
• -Z no-landing-pads
• -Z no-prepopulate-passes
• -Z no-vectorize-loops
• -Z no-vectorize-slp
• -Z soft-float
• -Z gen-crate-map
• -Z no-integrated-as

Things I don't think should be consolidated

• target (it's not just related to codegen)

We seem to be growing more of these every day, and I'm a little uneasy putting things in -Z flags, and I'm also a little uneasy about adding them as top-level flags.

Nominating, this is a backwards-compatibility issue with the compiler tools.

[pnkfelix]

Assigning P-backcompat-libs (sic, for "tools" like elsewhere), 1.0 blocker.