sim: Factor out NaN handling in floating point operations

2022-02-01  Faraz Shahbazker  <fshahbazker@wavecomp.com>

sim/common/ChangeLog:
	* sim-fpu.c (sim_fpu_op_nan): New.
	(sim_fpu_add): Factor out NaN operand handling with
	a call to sim_fpu_op_nan.
	(sim_fpu_sub, sim_fpu_mul, sim_fpu_div): Likewise.
	(sim_fpu_rem, sim_fpu_max, sim_fpu_min): Likewise.
	* sim-fpu.h (sim_fpu_op_nan): New declaration.