UefiCpuPkg/PiSmmCpuDxeSmm: eliminate conditional jump in IA32 SmmStartup()

UefiCpuPkg/PiSmmCpuDxeSmm: eliminate conditional jump in IA32 SmmStartup()

SMM emulation under both KVM and QEMU (TCG) crashes the guest when the
"jz" branch, added in commit d4d87596c11d ("UefiCpuPkg/PiSmmCpuDxeSmm:
Enable NXE if it's supported", 2018-01-18), is taken.

Rework the propagation of CPUID.80000001H:EDX.NX [bit 20] to IA32_EFER.NXE
[bit 11] so that no code is executed conditionally.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Ref: http://mid.mail-archive.com/d6fff558-6c4f-9ca6-74a7-e7cd9d007276@redhat.com
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
[lersek@redhat.com: XD -> NX code comment updates from Ray]
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
[lersek@redhat.com: mark QEMU/TCG as well in the commit message]