UefiCpuPkg/PiSmmCpuDxeSmm: Add volatile to mNumberToFinish

UefiCpuPkg/PiSmmCpuDxeSmm: Add volatile to mNumberToFinish

Add volatile qualifier to mNumberToFinish to prevent GCC 5.4
compiler from optimizing away required logic in ACPI S3 resume.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>