x86: unify the implementation of FPU traps

On 32 bits, we may suffer IRQ 13, or supposedly we might have a buggy
implementation which gives spurious trap 16.  We did not check for
this on 64 bits, but there is no reason we can't make the code the
same in both cases.  Furthermore, this is presumably rare, so do the
spurious check last, instead of first.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>