sim: Allow toggling of quiet NaN-bit semantics

IEEE754-1985 specifies the top bit of the mantissa as an indicator
of signalling vs. quiet NaN, but does not define the precise semantics.
Most architectures treat this bit as indicating quiet NaN, but legacy
(pre-R6) MIPS goes the other way and treats it as signalling NaN.

This used to be controlled by a macro that was only defined for MIPS.
This patch replaces the macro with a variable to track the current
semantics of the NaN bit and allows differentiation between older
(pre-R6) and and newer MIPS cores.

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

sim/common/ChangeLog:
	* sim-fpu.c (_sim_fpu): New.
	(pack_fpu, unpack_fpu): Allow reversal of quiet NaN semantics.
	* sim-fpu.h (sim_fpu_state): New struct.
	(_sim_fpu): New extern.
	(sim_fpu_quiet_nan_inverted): New define.

sim/mips/ChangeLog:
	* cp1.h (fcsr_NAN2008_mask, fcsr_NAN2008_shift): New.
	* mips.igen (check_fpu): Select default quiet NaN mode
	for legacy MIPS.
	* sim-main.h (SIM_QUIET_NAN_NEGATED): Remove.