Factor out lib32 generation to its own file. This is prep for a

Factor out lib32 generation to its own file. This is prep for a
similar Makefile.libsoft which will do the same for armv6 soft fp API
libraries in prep for pulling the trigger on moving to armv6 hard
float. Once there's two files, I'll work with bdrewery@ to merge the
two files as they are mostly the same. The high rate of churn for
Makefile* makes it quite difficult to make progress out of tree.

Differential Review: https://reviews.freebsd.org/D5566