Interpreter: fix arch-dep. behavior in NI=1 mode

PPC and x86 behave differently in non-IEEE mode (NI=1/FTZ):

For single-precision floating point operations with denormal inputs (and
frsp with a denormal output), PPC first flushes the input to zero, then
applies the current rounding mode while x86 does it the other way
around. At least this would explain what happens.

Ignoring the sign bit, the only range in which this affects the output
is [0x380ffffff0000000, 0x380fffffffffffff]. On PPC, a number in this
range results in zero while on x86 it results in 0x3810000000000000.
An example of this bug can be seen when running the frsp hwtest.

All credit goes to @FioraAeterna for finding the cause of this bug.

TODO: Investigate what ARM does.