OvmfPkg: introduce -D SMM_REQUIRE and PcdSmmSmramRequire
    
    This build time flag and corresponding Feature PCD will control whether
    OVMF supports (and, equivalently, requires) SMM/SMRAM support from QEMU.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>