gnu: cross-base: Apply Qualcomm's patches for Binutils and GCC.

This fixes compilation of the ath9k-htc firmware.  Before that, loading
it would result in "Target is unresponsive".  The patches come from
the ath9k-htc firmware source.

* gnu/packages/patches/ath9k-htc-firmware-binutils.patch,
  gnu/packages/patches/ath9k-htc-firmware-gcc.patch: New files.
* gnu-system.am (dist_patch_DATA): Add them.
* gnu/packages/cross-base.scm (package-with-patch): New procedure.
  (cross-binutils): Check whether TARGET starts with "xtensa-", and call
  'package-with-patch' when it does.
  (cross-gcc-patches): New procedure.
  (cross-gcc): Use it to add patches.