cmd/asm, math: add s390x floating point test instructions
Floating point test instructions allow special cases (NaN, ±∞ and
a few other useful properties) to be checked directly.
This CL adds the following instructions to the assembler:
* LTEBR - load and test (float32)
* LTDBR - load and test (float64)
* TCEB - test data class (float32)
* TCDB - test data class (float64)
Note that I have only added immediate versions of the 'test data
class' instructions for now as that's the only case I think the
compiler will use.