Rollup merge of #38459 - nagisa:rustbuild-exec, r=alexcrichton

Use exec for the wrapper on UNIXes

This not only avoids the small – and unnecessary – constant overhead for each compiler invocation,
but also helps somewhat by only having “correct” rustc processes to look for in `/proc/`.

This also makes the wrapper behave effectively as a regular exec wrapper its intended to be.

I also took liberty to change the fallback error code from `1` to `0xfe` (now only relevant on windows) so that when people complain about “compiler exited with code 254”, its obvious where the issue lies (wrapper losing the exit code somehow).

r? @alexcrichton

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