[MIPSR6] Use C-coded string ops on mips32r6/mips64r6

The existing assembler code uses deprecated lwl/lwr/swl/swr ops.
Replacing those with misalignment-forgiving lw/sw ops may
involve careful performance tuning.

Change-Id: I47a042f7b22b87d7d52e46c29c44b1db1ba8b693
1 file changed