bsd.compat.mk: Allow finding non-internal libraries

Currently only libexec/rtld-elf32 uses internal LIBC_NOSSP_PIC during
the build but it gets it directly from the objdir rather than a sysroot.
For example, /usr/obj/usr/src/amd64.amd64/obj-lib32/lib/libc/libc_nossp_pic.a.
We don't stage lib32 libraries in WORLDTMP/usr/lib32 and doing so doesn't
buy much. If we want to use a staged lib32 library then we need to look in
LIBCOMPATTMP where they were staged. For example if LIBC_PIC were wanted then
look for /usr/obj/usr/src/amd64.amd64/obj-lib32/tmp/usr/lib32/libc_pic.a.

Reported by: rlibby
Reviewed by: rlibby
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D27648