FreeBSD amd64 fixups

* Look for CPU named "amd64" as well as "x86_64"
* Don't use /dev/ptmx on FreeBSD
* On amd64 FreeBSD uses SIGSEGV, not SIGBUS
* Use MAP_FIXED to force allocations within 32-bits, it's the only way
* Need <sys/param.h> for SHMLBA
* The old offsetof() fix is no longer needed
* Preliminary work on instruction skipping
Merge pull request #40 from vasi/freebsd

FreeBSD/amd64