FreeBSD/amd64

Currently the macemu projects don't even compile on FreeBSD/amd64. This patch allows BasiliskII to compile and run well.

SheepShaver compiles ok with these fixups, but doesn't run successfully. It can't allocate low-memory in real-addressing mode, but I'm not sure why, since vmparam.h in the kernel has VM_MIN_ADDRESS set to zero. In direct-addressing mode, it SEGVs, and I haven't looked into it further. I should probably first check whether FreeBSD/i386 works, and whether direct-addressing works on other platforms.