- move the per-ABI subdir list into compatsubdir.mk. use the new
MAKEDIRTARGETENV support to force a new objdir for compat library builds,
and simply traverse over the normal src path once for each ABI to be
built. this eliminates all the shadow Makefile's for every library, and
allows multiple compat ABIs to be built for a single platform, such as
both o32 and n64 for mips64's default n32.
- compat/Makefile is merely a subdir provider now, and takes the real
information from archdirs.mk
