clang: Skip attempts to access /proc/self/fd

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.

Reviewed by: dim
Differential Revision: https://reviews.freebsd.org/D34684

(cherry picked from commit 30ed48b0a50a062415addcac216371a55e1f6a66)