Replace version & platform-compat shims in `compilers.C`

[jaraco]

Part of the compilers decoupling epic #5264 (Phase 1).

Remove the remaining version/platform-compat coupling in compilers.C:

• cygwin.py: distutils.version.LooseVersion → packaging.version
• Vendor _macos_compat.compiler_fixup and compat.consolidate_linker_args
• util.is_mingw, util.get_platform / get_host_platform → stdlib sysconfig / packaging.tags

Grouped PR, separate commits per concern. (The customize_compiler coupling is out of scope here — see the sysconfig investigation, #5264 Phase 1.)