UefiCpuPkg/PiSmmCpuDxeSmm: Add EFIAPI to CheckFeatureSupported()

The function CheckFeatureSupported() is used as an EFI_AP_PROCEDURE
in the MP Services Protocol service StartAllAPs().  Any function
used as an EFI_AP_PROCEDURE must use EFIAPI calling convention.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>