This avoids coupling the architecture directly to the platform,
allowing building on e.g. linux on architectures other than x86,
without having to explicitly specify USE_ASM=No while building
on those.
Later this will also facilitate enabling assembly optimizations
where applicable, e.g. arm assembly on linux.
Add platform-freebsd.mk makefile to allow building on FreeBSD.
OpenH264 builds and runs successfully on FreeBSD 9.2-RELEASE amd64, with both
the gcc and clang compilers.
Note that building i386 binaries with the native toolchain using the -m32 flag
on amd64 FreeBSD is unsupported and will not work, so ENABLE64BIT should always
be set to Yes when compiling on a FreeBSD amd64 system.
