OvmfPkg: Disable debugging for UNIXGCC to save space

OvmfPkg: Disable debugging for UNIXGCC to save space

UNIXGCC builds larger images than GCC44, and can have issues
fitting into the FD image. Therefore, when using UNIXGCC,
debug will be disabled by default.

The README file is updated with instructions for selectively
enabling debug for UNIXGCC.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13569 6f19259b-4bc3-4df7-8a09-765794883524