gnu: glibc: Update to 2.27.

gnu: glibc: Update to 2.27.

* gnu/packages/base.scm (glibc/linux): Update to 2.27.
[source](patches): Add 'glibc-hidden-visibility-ldconfig.patch' and
'glibc-2.27-git-fixes.patch'.
[native-inputs]: Add BISON.
[arguments]: Remove deprecated "--enable-add-ons" from #:configure-flags.
Also remove unneeded "libc_cv_ssp=no" and "libcv_ssp_strong=no".
(glibc-2.26): New public variable.
* gnu/packages/commencement.scm (glibc-final-with-bootstrap-bash)[native-inputs]:
Add BISON-BOOT0.
* gnu/packages/patches/glibc-2.27-git-fixes.patch,
gnu/packages/patches/glibc-hidden-visibility-ldconfig.patch: New files.
* gnu/local.mk (dist_patch_DATA): Register them.