MdePkg/BaseLib AARCH64: terminate stack frame list on stack switch

When switching to the DXE phase stack, set the frame pointer to zero so
that code walking the stack frame will not try to access stack frames
belonging to the old stack.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

MdePkg/Library/BaseLib/AArch64/SwitchStack.S

@@ -40,6 +40,7 @@ InternalSwitchStackAsm (
   );
 **/
 ASM_PFX(InternalSwitchStackAsm):
+    mov   x29, #0
     mov   x30, x0
     mov   sp, x3
     mov   x0, x1