Support `rustc-link-args` in order to properly pass `-pthread` and flags

[infinity0]

On rust-lang/rust#62117 it was discovered that certain C/C++ dependencies need to be linked with -pthread instead of -lpthread on certain platforms such as riscv64 and some arm cases.

Currently there is no way for a build.rs script to express this however. The closest thing is rustc-dylib-link-args which only applies when building a cdylib.

[joshtriplett]

#7811

[XieJiSS]

Just a reminder: This should be closed by #8441

[ehuss]

Closing, as it does sound like this is resolved, at least when using build scripts (docs).