[Support] Skip attempts to access /proc/self/fd on FreeBSD

In contrast to Linux it does not provide entries which can be readlinked
-- these are just regular files, not giving the expected outcome. That's
on top of procfs not being mounted by default to begin with.

This is probably the case on other BSDs as well, so I expect there will
be more ifdefs added down the road.

Reviewed By: emaste, dim

Differential Revision: https://reviews.llvm.org/D122545