glibc: update to 2.20 (does not build yet)

Getting "Error: CFI instruction used without previous .cfi_startproc"
in gconv_conf.c.

I skimmed release notes and found no nix-related stuff. Done this:
- drop patches included in release and make ld.so.preload apply,
- move out of the 2.19 directory as only one version is used
  and the renaming tends to cause merge problems,
- don't set 2.19 / 2.20 version on hurd as it was likely wrong.