Revert "Revert "Switch to NetBSD's strxfrm(3).""

This reverts commit 8793e7c7d21a0434d1b5e63364c88b2b125a3d29,
and fixes the build by building upstream NetBSD source as a
separate library that's then swallowed whole into libc_common.

Change-Id: I6c9317d8c48b5ccaf85a7b185bc07fb31176ff97
6 files changed