Detect hard/soft float in the same way as other ARM features
remove old cross-compilation stuff
We should be including HaskellMachRegs.h here, not RtsMachRegs.h

This was causing GenApply.hs to use the host architecture's register
settings rather than the target's, with the result that some
cross-compiled programs would crash.
\#undef REG_R[1-10] as a precaution
include "stg/MachRegs.h" -> include "MachRegs.h"

We were getting the installed compiler's MachRegs.h sometimes (in
GenApply.hs, as it turns out).  This probably caused some weird
effects...