Enable LLVM-based code generation for FreeBSD/amd64.
Allow "amd64" to be recognized as an x86_64 platform.
Re-enable DYNAMIC_GHC_PROGRAMS for FreeBSD.

All actively supported releases (8.4-RELEASE, 9.2-RELEASE and the
upcoming 10.0-RELEASE) now support resolution of $ORIGIN properly.
Fix #8451

On FreeBSD, /usr/lib has to be added to the library path on linking when
libthr is needed but -nostdlib is used (which is the case when the -prof
and -threaded flags are combined).