UefiCpuPkg/PiSmmCpuDxeSmm: remove *.S and *.asm assembly files

All edk2 toolchains use NASM for compiling X86 assembly source code. We
plan to remove X86 *.S and *.asm files globally, in order to reduce
maintenance and confusion:

http://mid.mail-archive.com/4A89E2EF3DFEDB4C8BFDE51014F606A14E1B9F76@SHSMSX104.ccr.corp.intel.com
https://lists.01.org/pipermail/edk2-devel/2018-March/022690.html
https://bugzilla.tianocore.org/show_bug.cgi?id=881

Let's start with UefiCpuPkg/PiSmmCpuDxeSmm: remove the *.S and *.asm
dialects (both Ia32 and X64) of the SmmInit, SmiEntry, SmiException and
MpFuncs sources.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=866
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Andrew Fish <afish@apple.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>