Doesn't quite match the libc code directly because it uses `[gs:eax]`
with both having the sign bit set and we can't deal with that with ASM
tests. So match the behaviour in a different way.
When the source arguments for LoadMem/StoreMem have bit 31 set then they
are incorrectly sign extending in some instances.
Detected this when testing #3421 but I don't have a proper fix for it.