PPC/s390: [cleanup] Use Vector::begin instead of Vector::start

Port 08756826

Original Commit Message:

    Port 4b0f9c85

    Original Commit Message:

        Our {Vector} template provides both {start} and {begin} methods. They
        return exactly the same value. Since the {begin} method is needed for
        iteration, and is also what standard containers provide, this CL
        switches all uses of the {start} method to use {begin} instead.

        Patchset 1 was auto-generated by using this clang AST matcher:
            callExpr(
                callee(
                  cxxMethodDecl(
                    hasName("start"),
                    ofClass(hasName("v8::internal::Vector")))
                ),
                argumentCountIs(0))

        Patchset 2 was created by running clang-format. Patchset 3 then
        removes the now unused {Vector::start} method.

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

Change-Id: Ief052e7655ede161504cf058eddd81714e6e5929
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1590168
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61142}

src/objects/bigint.cc

@@ -1956,7 +1956,7 @@ MaybeHandle<BigInt> BigInt::FromSerializedDigits(
 #elif defined(V8_TARGET_BIG_ENDIAN)
   digit_t* digit = reinterpret_cast<digit_t*>(digits);
   const digit_t* digit_storage =
-      reinterpret_cast<const digit_t*>(digits_storage.start());
+      reinterpret_cast<const digit_t*>(digits_storage.begin());
   for (int i = 0; i < bytelength / kDigitSize; i++) {
     *digit = ByteReverse(*digit_storage);
     digit_storage++;