V8: do not use wide reads in CopyCharsUnsigned

Fixes segfault in 32bit SmartOS when built with GCC 4.9.

This is the first of two backports from upstream v8:
1. v8/v8@90dc5c9
2. v8/v8@7cb82a7

Original commit message:

  Do not use wide reads in CopyCharsUnsigned.

  R=jkummerow@chromium.org
  BUG=chromium:412967
  LOG=Y

  Review URL: https://codereview.chromium.org/566583002

  git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23876 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

V8 issue: https://code.google.com/p/chromium/issues/detail?id=412967

Fixes #25281

Reviewed-By: Julien Gilli <julien.gilli@joyent.com>
PR-URL: #25556

Author: hashseed

deps/v8/src/utils.h

@@ -1351,20 +1351,10 @@ template <typename sourcechar, typename sinkchar>
 void CopyCharsUnsigned(sinkchar* dest, const sourcechar* src, int chars) {
   sinkchar* limit = dest + chars;
 #ifdef V8_HOST_CAN_READ_UNALIGNED
-  if (sizeof(*dest) == sizeof(*src)) {
-    if (chars >= static_cast<int>(kMinComplexMemCopy / sizeof(*dest))) {
-      MemCopy(dest, src, chars * sizeof(*dest));
-      return;
-    }
-    // Number of characters in a uintptr_t.
-    static const int kStepSize = sizeof(uintptr_t) / sizeof(*dest);  // NOLINT
-    DCHECK(dest + kStepSize > dest);  // Check for overflow.
-    while (dest + kStepSize <= limit) {
-      *reinterpret_cast<uintptr_t*>(dest) =
-          *reinterpret_cast<const uintptr_t*>(src);
-      dest += kStepSize;
-      src += kStepSize;
-    }
+  if ((sizeof(*dest) == sizeof(*src)) &&
+      (chars >= static_cast<int>(kMinComplexMemCopy / sizeof(*dest)))) {
+    MemCopy(dest, src, chars * sizeof(*dest));
+    return;
   }
 #endif
   while (dest < limit) {