stand: push LIBC_SRC up into defs.mk

Other parts of stand/ that don't use libsa will need to grab bits from libc
shortly. Push LIBC_SRC up to defs.mk in advance of this so that they can use
it, and rename it to LIBCSRC to match the convention of the rest of the *SRC
variables in this file.

Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21282