MdePkg/BaseLib: Add support for the XGETBV instruction
    
    BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198
    
    Under SEV-ES, a CPUID instruction requires the current value of the XCR0
    register. In order to retrieve that value, the XGETBV instruction needs
    to be executed.
    
    Provide the necessary support to execute the XGETBV instruction.
    
    Cc: Michael D Kinney <michael.d.kinney@intel.com>
    Cc: Liming Gao <liming.gao@intel.com>
    Reviewed-by: Liming Gao <liming.gao@intel.com>
    Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
    Regression-tested-by: Laszlo Ersek <lersek@redhat.com>