Document environment variables used by programs rustc invokes underneath (linkers, etc)

[boris-kolpackov]

This is related to issue #16330: rustc invokes other programs underneath (most notably, a linker) which recognize environment variables of their own. A prominent example would be the LIB variable that is recognized by Microsoft link.exe (and lld-link) which is what, as I understand, is used by rustc on Windows.

Having an accurate list of environment variables that can affect the result of compilation is important to build systems that track changes to the environment.

[boris-kolpackov]

Another prominent example would be MACOSX_DEPLOYMENT_TARGET (and a slew of other *_DEPLOYMENT_TARGET variables) recognized by ld64 on Mac OS.

[jonas-schievink]

Since the environment variables used by the linker can vary depending on its version, the best way to do this would probably to link to the linker's documentation

[madsmtm]

The Apple-specific *_DEPLOYMENT_TARGET and SDKROOT were documented in #124772.