[mips] Expand BuildPairF64 to a spill and reload when the O32 FPXX ABI is enabled and mthc1 and dmtc1 are not available (e.g. on MIPS32r1)
This prevents the upper 32-bits of a double precision value from being moved to the FPU with mtc1 to an odd-numbered FPU register. This is necessary to ensure that the code generated executes correctly regardless of the current FPU mode.
MIPS32r2 and above continues to use mtc1/mthc1, while MIPS-IV and above continue to use dmtc1.