Alter bsd.compat.mk to set MACHINE and MACHINE_ARCH when included
directly so MD paths in Makefiles work. In the process centralize
setting them in LIBCOMPATWMAKEENV.
Alter .PATH and CFLAGS settings in work when the Makefile is included.
While here only support LIB32 on supported platforms rather than always enabling it and requiring users of MK_LIB32 to filter based TARGET/MACHINE_ARCH.
The net effect of this change is to make Makefile.libcompat only build