Use more information from `rustc`'s target spec

[madsmtm]

There's a lot more useful information exposed in rustc's target spec that cc could use after #1225. Opening this issue to track the progress on using those.

Specifically:

• llvm-target: #1252
• llvm-abiname
• target-endian
• linker
• pre-link-objects/post-link-objects/pre-link-objects-fallback/crt-static-allows-dylibs/etc. for better -Clink-self-contained?
• link-env/link-env-remove
• cpu? See also #268
• dynamic-linking (for diagnostics?)
• frame-pointer
• default-dwarf-version
• position-independent-executables/static-position-independent-executables
• panic-strategy (for -fno-exceptions)
• eh-frame-header?
• Atomics (cfg!(target_has_atomic = "..."))?
• Whether thread locals are available (cfg!(target_thread_local))?

Note that we still have to implement fallbacks for each of these for custom target specs.