MIPS: unwind, don't save/restore hi/lo for R6

HI/LO registers have been removed in MIPSr6. Save and restore them only for pre-R6.

We keep the memory space for the registers so that we can use the same register indexes for
r6 and pre-r6.

Fixes: #60682

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D156283