Don't create subprocesses for the `rust` command

Instead link against the built libraries and directly invoke those.
auto merge of #6438 : alexcrichton/rust/issue-5387, r=thestinger

Instead link against the built libraries and directly invoke those.

Closes #5387
Correct #[always_inline] -> #[inline(always)] and __attribute((...)) -> __attribute__((...)).
auto merge of #6447 : luqmana/rust/6446, r=thestinger

#6446