Add FreeBSD support to qv4targetplatform_p.h

Currently, that code only works on Linux, Windows, Mac OS X
and QNX on x86 and x86_64 processors.

Change-Id: I07f2cb4cb9b8556d4bcd681c6a1f997dc3897901
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>