Richard,

This patch implements the target hook TARGET_FN_OTHER_HARD_REG_USAGE (posted 
here: http://gcc.gnu.org/ml/gcc-patches/2013-03/msg01318.html) for MIPS, to 
address the issue that $6 is sometimes used in split calls.

Build and reg-tested on MIPS.

OK for stage1?

Thanks,
   - Tom