[x86 setup] Correct the SMAP check for INT 0x15, AX=0xe820

The e820 probe code was checking %edx, not %eax, for the SMAP
signature on return.  This worked on *almost* all systems, since %edx
still contained SMAP from the call on entry, but on a handful of
systems it failed -- plus, we would have missed real mismatches.

The error output is "=d" to make sure gcc knows %edx is clobbered
here.

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