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

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