PPC/s390: [Liftoff] Fix stack slot initialization on arm and arm64

Port 7d09b270

Original Commit Message:

    It turns out that because we are *subtracting* from fp, we need to
    *subtract less* to get a higher address. Who knew.

R=clemensb@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: Ide47f62b5fbfd309a2892fcd934175db7e390a8c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1864586Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#64340}

src/wasm/baseline/ppc/liftoff-assembler-ppc.h

@@ -174,7 +174,7 @@ void LiftoffAssembler::FillStackSlotsWithZero(uint32_t index, uint32_t count) {
     push(r4);
     push(r5);
     subi(r4, fp, Operand(liftoff::GetStackSlotOffset(last_stack_slot)));
-    subi(r5, fp, Operand(liftoff::GetStackSlotOffset(index) + kStackSlotSize));
+    subi(r5, fp, Operand(liftoff::GetStackSlotOffset(index) - kStackSlotSize));
 
     Label loop;
     bind(&loop);

src/wasm/baseline/s390/liftoff-assembler-s390.h

@@ -173,7 +173,7 @@ void LiftoffAssembler::FillStackSlotsWithZero(uint32_t index, uint32_t count) {
     push(r3);
     push(r4);
     SubP(r3, fp, Operand(liftoff::GetStackSlotOffset(last_stack_slot)));
-    SubP(r4, fp, Operand(liftoff::GetStackSlotOffset(index) + kStackSlotSize));
+    SubP(r4, fp, Operand(liftoff::GetStackSlotOffset(index) - kStackSlotSize));
 
     Label loop;
     bind(&loop);