gnu: base: Add glibc-hurd and hurd-minimal.

* gnu/packages/base.scm (glibc/hurd, glibc/hurd-headers): New variables.
* gnu/packages/hurd.scm (hurd-minimal): New variable.
* gnu/packages/patches/glibc-hurd-extern-inline.patch: New file.
* gnu/packages/patches/libpthread-glibc-preparation.patch: New file.
* gnu-system.am (dist_patch_DATA): Add them.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>
Co-authored-by: Mark H Weaver <mhw@netris.org>