Adds the 8 most essential floating-point arithmetic instructions:
fadd.s/d, fsub.s/d, fmul.s/d, fdiv.s/d, for both single and double
precision. These complement the existing fsgnj/fmax/fmin/fsqrt set.
Comparison (feq/flt/fle) and conversion (fcvt) instructions remain
as future work.
