mips: fix testsuite build for O32 FPXX ABI on pre-R2 CPU

mips: fix testsuite build for O32 FPXX ABI on pre-R2 CPU

On MIPS when the toolchain is using the O32 FPXX ABI, the testsuite
fails to build for pre-R2 CPU.

It assumes that it is possible to use the -mfp64 option to build
tst-abi-fp64amod and tst-abi-fp64mod, while this requires a CPU which
supports the mfhc1 and mthc1 instructions, ie at least a R2 CPU:

  error: '-mgp32' and '-mfp64' can only be combined if the target
  supports the mfhc1 and mthc1 instructions

The same way it assumes that it is possible to use the -modd-spreg option
to build tst-abi-fpxxomod and tst-abi-fp64mod, while this requires at
least a R1 CPU:

  warning: the 'mips2' architecture does not support odd
  single-precision registers

This patches changes that by checking the usability of -mfp64 and
-modd-spreg options in configure, and disable those tests when they can
not be used.