I really should do this when less tired; I keep breaking one platform
while fixing another. I've also fixed some Windows issues with tcctest
since Windows printf() uses different format flags to those on Linux,
and removed some conditional compilation tests in tcctest since they
now should work.
I removed the XMM6/7 registers from the register list because they are not used
on Win64 however they are necessary for parameter passing on x86-64. I have now
restored them but not marked them with RC_FLOAT so they will not be used except
for parameter passing.