sys-libs/glibc: Add new bootstrap files from stages, bug 647070

sys-libs/glibc: Add new bootstrap files from stages, bug 647070

See https://github.com/gentoo/gcc-multilib-bootstrap/ for the source.
This is as good as we can do at the moment. Tested by building an
x32 glibc on a normal amd64 system.

For the glibc-2.26 stabilization we still need new files for PowerPC.

Current status:
* amd64, x32, x86: Files from gcc-6.4
* s390, s390x: Files from gcc-5.4 (does this work?)
* ppc, ppc64: no files, since only gcc-4.9 available
* n32, n64, o32: no files, last mips stages are years old, not useful

Bug: https://bugs.gentoo.org/647070
Package-Manager: Portage-2.3.31, Repoman-2.3.9