rtld-elf: Migrate from COMPAT_32BIT to generic COMPAT_libcompat/LIBCOMPAT

We still have a tiny amount of libcompat-specific code in rtld_paths.h,
but it's been deduplicated as much as possible, and in future we may
wish to just push these variables down to the few consumers of them and
make them use the double-underscore variants with a libcompat argument
rather than give them names here.

See commit 8fad2cda93c7 ("bsd.compat.mk: Provide new CPP and sub-make
variables") for the context behind this change.

Reviewed by: kib, brooks, jhb
Differential Revision: https://reviews.freebsd.org/D40925