Reland "[snapshot] Align internal snapshot data"

This is a reland of 4f9d7a94

Original change's description:
> [snapshot] Align internal snapshot data
>
> When the snapshot blob is not aligned properly, loading it can cause a
> crash on platforms such as arm.
>
> This was exposed by a SIGBUS/BUS_ADRALN crash on arm when accessing
> the blob_data symbol (declared as a byte array) through a reinterpret
> cast to uintptr_t in an internal snapshot build.
>
> Thanks to florian.dold@gmail.com for the initial patch.
>
> Bug: v8:9171
> Change-Id: I99b071dec3733416f2f01b58a770e30d8f2dcdf2
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1582402
> Commit-Queue: Dan Elphick <delphick@chromium.org>
> Auto-Submit: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Dan Elphick <delphick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#61000}

Tbr: delphick@chromium.org
Bug: v8:9171
Change-Id: I36f53647ff5c45bcc512147f082fdd069723175d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1587377Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61062}

src/snapshot/mksnapshot.cc

@@ -87,7 +87,8 @@ class SnapshotFileWriter {
 
   static void WriteSnapshotFileData(FILE* fp,
                                     const i::Vector<const i::byte>& blob) {
-    fprintf(fp, "static const byte blob_data[] = {\n");
+    fprintf(fp,
+            "alignas(kPointerAlignment) static const byte blob_data[] = {\n");
     WriteBinaryContentsAsCArray(fp, blob);
     fprintf(fp, "};\n");
     fprintf(fp, "static const int blob_size = %d;\n", blob.length());

src/snapshot/serializer-common.h

@@ -375,8 +375,14 @@ class Checksum {
     // Fletcher's checksum. Modified to reduce 64-bit sums to 32-bit.
     uintptr_t a = 1;
     uintptr_t b = 0;
-    const uintptr_t* cur = reinterpret_cast<const uintptr_t*>(payload.start());
+    // TODO(jgruber, v8:9171): The following DCHECK should ideally hold since we
+    // access payload through an uintptr_t pointer later on; and some
+    // architectures, e.g. arm, may generate instructions that expect correct
+    // alignment. However, we do not control alignment for external snapshots.
+    // DCHECK(IsAligned(reinterpret_cast<intptr_t>(payload.start()),
+    //                  kIntptrSize));
     DCHECK(IsAligned(payload.length(), kIntptrSize));
+    const uintptr_t* cur = reinterpret_cast<const uintptr_t*>(payload.start());
     const uintptr_t* end = cur + payload.length() / kIntptrSize;
     while (cur < end) {
       // Unsigned overflow expected and intended.