ArmPlatformPkg/PrePeiCore: seed temporary stack before entering PEI core

DEBUG builds of PEI code will print a diagnostic message regarding
the utilization of temporary RAM before switching to permanent RAM.
For example,

  Total temporary memory:    16352 bytes.
    temporary memory stack ever used:       4820 bytes.
    temporary memory heap used for HobList: 4720 bytes.

Tracking stack utilization like this requires the stack to be seeded
with a known magic value, and this needs to occur before entering C
code, given that it uses the stack. Currently, only Nt32Pkg appears
to implement this feature, but it is useful nonetheless, so let's
wire it up for PrePeiCore.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

ArmPlatformPkg/PrePeiCore/Arm/PrePeiCoreEntryPoint.asm

@@ -79,6 +79,14 @@ _PrepareArguments
 
 _SetupPrimaryCoreStack
   mov   sp, r1
+  mov32 r8, FixedPcdGet64 (PcdCPUCoresStackBase)
+  mov32 r9, FixedPcdGet32 (PcdInitValueInTempStack)
+  mov   r10, r9
+  mov   r11, r9
+  mov   r12, r9
+0:stm   r8!, {r9-r12}
+  cmp   r8, r1
+  blt   0b
   b     _PrepareArguments
 
 _NeverReturn