This ports the fpadd751_asm and fpsub751_asm functions from the MSR
implementation to Go, and adds property testing and benchmarks. I chose these
functions to start because they use no stack, so there's no need to interact
with Go's stack handling.
Some care and trickery is required because Go's assembler misassembles `MOVQ
$0, AX` into `xor eax, eax`, which destroys the carry flags. Otherwise the
assembly is essentially similar (i.e., easily diff'able) compared to the MSR
implementation.