PPC: [wasm-c-api] Add tests and fixes

Port f72c844a

Original Commit Message:

    Port f5ab7d38

    Port 65f3861e

    Original Commit Message:

        In a new test suite: "wasm-api-tests", using a new binary "wasm_api_tests",
        powered by gtest/gmock (like unittests).
        Also fix a bunch of issues that these tests uncovered, mostly to ensure
        that the stack is walkable.

R=miladfar@ca.ibm.com, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: I565fba1a2437eb49be0c5247740f3fcac7b071db
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1775723Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#63460}

src/compiler/backend/ppc/code-generator-ppc.cc

@@ -1024,13 +1024,14 @@ CodeGenerator::CodeGenResult CodeGenerator::AssembleArchInstruction(
       Label start_call;
       bool isWasmCapiFunction =
           linkage()->GetIncomingDescriptor()->IsWasmCapiFunction();
-      constexpr int offset = 12;
+      constexpr int offset = 9 * kInstrSize;
       if (isWasmCapiFunction) {
-        __ mflr(kScratchReg);
+        __ mflr(r0);
         __ bind(&start_call);
-        __ LoadPC(r0);
-        __ addi(r0, r0, Operand(offset));
-        __ StoreP(r0, MemOperand(fp, WasmExitFrameConstants::kCallingPCOffset));
+        __ LoadPC(kScratchReg);
+        __ addi(kScratchReg, kScratchReg, Operand(offset));
+        __ StoreP(kScratchReg,
+                  MemOperand(fp, WasmExitFrameConstants::kCallingPCOffset));
         __ mtlr(r0);
       }
       if (instr->InputAt(0)->IsImmediate()) {
@@ -1040,11 +1041,11 @@ CodeGenerator::CodeGenResult CodeGenerator::AssembleArchInstruction(
         Register func = i.InputRegister(0);
         __ CallCFunction(func, num_parameters);
       }
-      // TODO(miladfar): In the above block, r0 must be populated with the
-      // strictly-correct PC, which is the return address at this spot. The
-      // offset is set to 12 right now, which is counted from where we are
-      // binding to the label and ends at this spot. If failed, replace it it
-      // with the correct offset suggested. More info on f5ab7d3.
+      // TODO(miladfar): In the above block, kScratchReg must be populated with
+      // the strictly-correct PC, which is the return address at this spot. The
+      // offset is set to 36 (9 * kInstrSize) right now, which is counted from
+      // where we are binding to the label and ends at this spot. If failed,
+      // replace it with the correct offset suggested. More info on f5ab7d3.
       if (isWasmCapiFunction)
         CHECK_EQ(offset, __ SizeOfCodeGeneratedSince(&start_call));