x86: prioritize the FPU traps for the error code

In the case of multiple FPU errors, prioritize the error codes,
instead of returning __SI_FAULT, which ends up pushing a 0 as the
error code to userspace, a POSIX violation.

For i386, we will simply return if there are no errors at all; for
x86-64 this is probably a "can't happen" (and the code should be
unified), but for this patch, return __SI_FAULT|SI_KERNEL if this ever
happens.

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