Parse --cc and --cross-prefix earlier and use CC to determine cpu and host

Parse --cc and --cross-prefix earlier and use CC to determine cpu and host

We have been relying on uname to determine the host cpu architecture and
operating system.  This is totally broken for cross compilation.  It was
workable in the past because you can manually override both settings but after
the host USB passthrough refactoring, cross host builds were broken.

This moves the parsing of --cc and --cross-prefix to before the probes for cpu
and host.  Complation testing is used to determine the host and CPU types.  I've
only added checks for i386, x86_64, Linux, and Windows since these are the only
platforms I have access to for testing.  Everything else falls back to uname.

It should be relatively easy to add the right checks for other platforms and
eliminate uname altogether.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6141 c046a42c-6fe2-441c-8c8c-71466251a162