Parallelise flag support check in try_get_compiler

[NobodyXu]

A while ago, I was thinking of parallelising this to speed it up.

If you'd like to do this, I recommend creating a new fn is_flags_supported.

You can take a look at compile_objects, if you are interested in doing so

Originally posted by @NobodyXu in https://github.com/rust-lang/cc-rs/pull/1050#discussion_r1589827852