On amd64 machines without the NX feature enabled, we can't distinguish

On amd64 machines without the NX feature enabled, we can't distinguish
between page faults as a result of instruction fetches or normal data
access.  Handle this in the same way as we do on landisk: if handling
the fault with access type PROT_READ fails, retry with PROT_EXEC.
Fortunately we know whether NX is enabled or nor so only do this when
it isn't.  Nobody should be running an amd64 machine without NX!

ok deraadt@, miod@