Strip RCSID() and SCCSID() strings by default when building libc.

This is in preparation for changes to update the various ID strings in
libc's source. CSRG ID strings will use SCCSID() and there are some
existing uses of RCSID() for NetBSD ID strings already. These are
generally under either an explicit #if 0 or an #ifdef LIBC_SCCS so are
off by default and this change preserves that existing behavior.

Differential Revision: https://reviews.freebsd.org/D15830