UefiCpuPkg: Use FINIT instead of hex values

https://bugzilla.tianocore.org/show_bug.cgi?id=560

Update X64 NASM file to match IA32 NASM file
and use FINIT instruction instead of hand
assembled hex values for the FINIT instruction.

Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>