target-sh4: implement FPU exceptions

target-sh4: implement FPU exceptions

FPU exception support where not implemented on SH4. Implement them by
clearing the softfloat exceptions flags before an FP instruction (the
SH4 FPU also clear them before an instruction), and calling a function
to update the FPSCR register after an FP instruction. This function
update the corresponding FPSCR bits (both flags and cumulative flags)
and trigger exception if enabled.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>