MdePkg/BaseLib: Add support for the VMGEXIT instruction

MdePkg/BaseLib: Add support for the VMGEXIT instruction

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198

VMGEXIT is a new instruction used for Hypervisor/Guest communication when
running as an SEV-ES guest. A VMGEXIT will cause an automatic exit (AE)
to occur, resulting in a #VMEXIT with an exit code value of 0x403.

Since SEV-ES is only supported in X64, provide the necessary X64 support
to execute the VMGEXIT instruction, which is coded as "rep vmmcall". For
IA32, since "vmmcall" is not supported in NASM 32-bit mode and VMGEXIT
should never be called, provide a stub implementation that is identical
to CpuBreakpoint().

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>