Recognize amd64 installs as 86_64 arches.

FreeBSD calls x86_64 "amd64", so this allows allows FreeBSD installs to
have AVX/XOP support without any significant modifications -- it should
be noted that the base install of gcc doesn't actually have support,
but a later version from ports can be installed by the user and chosen
via e.g., `CC=/usr/local/bin/gcc48`.