[libunwind][mips] Guard accumulator registers

Mipsr6 does not possess HI and LO accumulator registers, adjust validRegister functions to respect that.

Differential Revision: https://reviews.llvm.org/D50244


git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@339849 91177308-0d34-0410-b5e6-96231b3b80d8