Strip __RCSID() and __SCCSID() strings by default when building libc.

[jhb]

Summary

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.

Test Plan

• build a libc with this change (and some other changes in libc sources to update ID strings) and verify the .comment contents are still the same