[MIPSR6] Add mips64r6 and mips32r6 targets

Add mips64r6 target and corresponding mips32r6 target.
Defaults remain as mips64r2 and mips32r2.

Apply -FP64A codegen subsetting to mips32r6 only.
Access FR=0 odd-numbered 32-bit float regs only via
double-prec even-numbered regs, not by single-prec ops.

Change-Id: I1740a6c658304b6c41242be58d68753e6f171658