Move Makefile.lib32 to Makefile.libcompat and generalize it.

This is in preparation for LIBSOFT.

This file only supports *1* LIBCOMPAT value currently and must be capitalized.
In Makefile.libcompat given LIBCOMPAT=FOO there can be values set for
LIBFOOCFLAGS, LIBFOOCPUFLAGS, LIBFOOWMAKEENV, LIBFOOWMAKEFLAGS, LIBFOOCPUFLAGS,
and LIBFOODTRACE. These will have the standard cross-build values appended
onto them.

This could be extended to support multiple libcompat libraries in the future
once there is a need.

Reviewed by: imp
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D5612