llvm: Support: don't block signals around close if it can be avoided

Signal blocking originally showed up in 51c2afc4b65b2782 ("Support:
Don't call close again if we get EINTR"), but it was overzealous --
there are systems where the error is known to be fine.

This commit elides signal blocking for said systems (the list is
incomplete though).

Note close() can still fail for other reasons (like ENOSPC), in which
case an error will be returned while the fd slot is cleared up.

Reviewed by: dim
Differential Revision: https://reviews.freebsd.org/D42984