msun: LIBCSRCDIR is too fragile, use ${SRCTOP}/lib/libc instead

LIBCSRCDIR is defined in bsd.libnames.mk, which is read in later in the
Makefile than the line:

.if exists(${LIBCSRCDIR}/${MACHINE_ARCH})

so we test to see if /${MARCHIN_ARCH} exists which it usually doesn't
(but did for me since I mounted 13.2R SD image there). Move to defining
our own LIBC_SRCTOP in terms of SRCTOP to treat these uniformily.

Sponsored by: Netflix
Reviewed by: sjg
Differential Revision: https://reviews.freebsd.org/D41661