use INT3 instead of NOP for x86 binary padding

We should never end up executing the inter-function padding, so we
are better off faulting than silently carrying on to whatever function
happens to be next.

Note that LLD will soon do this by default (although it currently pads
with zeros).

Reviewed by: dim, kib
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10047