Make sure JIT architectures lists are in sync

We have two lists: in qv4global_p.h and qv4targetplatform_p.h. This commit blacklists
OSes on x86 and x86_64 where JIT is not supported, improves support for FreeBSD and
adds cross-references between these two files.

Change-Id: Id3715a2ab717186e510a54e5a548dfa22120cd87
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>