gnu: Add toolchain support for 'armhf-linux'.

gnu: Add toolchain support for 'armhf-linux'.

Based on preliminary work by John Darrington <john@darrington.wattle.id.au>.

* gnu/packages/cross-base.scm (xgcc-armhf): New variable.
* gnu/packages/gcc.scm (gcc-configure-flags-for-triplet): Add armhf case.
  (gcc-4.7)[pre-configure]: Add gcc/config/*/linux-eabi.h to the list
  of files in which to patch GLIBC_DYNAMIC_LINKER.
* gnu/packages/bootstrap.scm (glibc-dynamic-linker): Add armhf case.
* guix/utils.scm (gnu-triplet->nix-system, nix-system->gnu-triplet):
  Add armhf cases.