UefiCpuPkg/MpInitLib: Change AP Index variable name.

Original AP index variable name not well express the meaning
of the variable. Also this name is better used in later patch.
So change the variable name for better understanding.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jeff Fan <vanjeff_919@hotmail.com>

UefiCpuPkg/Library/MpInitLib/Ia32/MpFuncs.nasm

@@ -130,7 +130,7 @@ TestLock:
     jz         TestLock
 
     mov        ecx, esi
-    add        ecx, NumApsExecutingLocation
+    add        ecx, ApIndexLocation
     inc        dword [ecx]
     mov        ebx, [ecx]
 
@@ -200,7 +200,7 @@ CProcedureInvoke:
     mov        eax, ASM_PFX(InitializeFloatingPointUnits)
     call       eax               ; Call assembly function to initialize FPU per UEFI spec
 
-    push       ebx               ; Push NumApsExecuting
+    push       ebx               ; Push ApIndex
     mov        eax, esi
     add        eax, LockLocation
     push       eax               ; push address of exchange info data buffer