Add link-flags-ld emit option.

[rillian]

Attempt to address #31471. r? @alexcrichton

• Assumes unix-style ldflags.
• Doesn't propagate OutputFilenames, so the default output filename doesn't work.
• Wasn't sure where/how to add a test for this run-make and check if a file is produced?

[bors]

☔ The latest upstream changes (presumably #31882) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

Thanks @rillian! I think we may want to document this as well somewhere. I guess the best place may be... the man page? I don't think we have exhaustive docs in the reference for this right now...

Also in terms of tests, yeah we'll want to add some tests for this along the lines of:

• Make sure the file is emitted
• Make sure -o is respected
• Make sure --emit foo=bar is respected
• Ensure warnings aren't printed if this emit type is specified

etc

You should be able to add a test by just creating a directory in src/test/run-make, adding a Makefile, and then just doing whatever shell commands you need to run the tests. There should be plenty of examples in sibling tests.

cc @rust-lang/tools, this is also adding what would essentially be a new insta-stable emission type of the compiler.

[rillian]

Thanks for the feedback, @alexcrichton. I've rebased around the bit-rot. Will add tests and documentation.

[alexcrichton]

Closing due to inactivity, but feel free to resubmit with the tests + docs!