Fix building rustc on and for musl hosts.

[mixi]

This fixes all problems I had when trying to compile rustc on a musl-based distribution (with crt-static = false in config.toml).

This is a fixed version of what ended up being #50105, making it possible to compile rustc on musl targets.

The differences to the old (now merged and subsequently reverted) pull request are:

• The commit (6d9154a) that caused the regression for which the original commits were reverted in Revert #50105 until regression is fixed #50709 is left out. This means the corresponding bug link: failed to static link to c++ library when global variable is used #36710 is still not fixed with +crt-static.
• The test for issue 36710 is skipped for musl targets (until the issue is properly fixed).
• Building cargo-vendor if crt-static = false is needed was broken (cargo-vendor links to some shared libraries if they exist on the system and this produces broken binaries with +crt-static)

CC @alexcrichton