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. https://github.com/v8/v8-git-mirror/commit/90dc5c9e66ef1778cb8425c43c2f37b6799260c3
2. https://github.com/v8/v8-git-mirror/commit/7cb82a76b46ef18f8e05d40d4fe13b3410d446e4

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: https://github.com/joyent/node/pull/25556