math_efp.c: Fixed SPE data type conversion failure

[Shan Hai]

The following test case failed on Powerpc sbc8548 with CONFIG_SPE

static float fm;
static signed int si_min = (-2147483647 - 1);
static unsigned int ui;
int main()
{
        fm = (float) si_min; ;
        ui = (unsigned int)fm;
        printf("ui=%d, should be %d\n", ui, si_min);

        return 0;
}
Result: ui=-1, should be -2147483648

The reason is failure to emulate the minus float to unsigned integer conversion 
instruction in the SPE driver.