We should avoid including the whole of softfloat headers in cpu.h and
explicitly include it only where we will be calling softfloat
functions. We can use the -types.h in cpu.h for the few bits that are
global. We also move the restore_snan_bit_mode into internal.h and
include -helpers.h there.
Backports commit 502700d0674919309a19bfd016ea0680c6b7747d from qemu