target-mips: raise RI exceptions when FIR.PS = 0

target-mips: raise RI exceptions when FIR.PS = 0

64-bit paired-single (PS) floating point data type is optional in the
pre-Release 6.
It has to raise RI exception when PS type is not implemented. (FIR.PS = 0)
(The PS data type is removed in the Release 6.)
Loongson-2E and Loongson-2F don't have any implementation field in
FCSR0(FIR) but do support PS data format, therefore for these cores RI will
not be signalled regardless of PS bit.

Signed-off-by: Yongbok Kim <[email protected]>
Reviewed-by: Leon Alrae <[email protected]>
Reviewed-by: Aurelien Jarno <[email protected]>
Signed-off-by: Leon Alrae <[email protected]>