OvmfPkg: Add XCODE5 statements to fix build break

OvmfPkg: Add XCODE5 statements to fix build break

https://bugzilla.tianocore.org/show_bug.cgi?id=559

The XCODE5 tool chain has a FAMILY of GCC.  The
GCC statements in the [BuildOptions] section add
flags that are not compatible with XCODE5.  Add
empty XCODE5 statements in [BuildOptions] sections
to prevent the use of the GCC flags in XCODE5
builds.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Andrew Fish <afish@apple.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>