fix -Wunused-but-set-variable for gcc-4.6 on x64

fix -Wunused-but-set-variable for gcc-4.6 on x64

* src/third_party/valgrind/valgrind.h: Update from upstream valgrind
  r11899, so as to get around some unused value warnings.  Also adds
  support for darwin.

  This version of valgrind.h differs from the original in that all
  instances of "unsigned long long int" have been replaced with
  "uint64_t", as the former is not allowed in ISO C++ 89.

  See https://bugs.kde.org/show_bug.cgi?id=211926 for the upstream bug
  report.

* src/x64/cpu-x64.cc:
* src/builtins.cc:
* src/conversions-inl.h:
* src/debug.cc:
* src/frames.cc:
* src/full-codegen.cc:
* src/jsregexp.cc:
* src/objects.cc:
* src/parser.cc:
* src/platform-linux.cc:
* src/x64/code-stubs-x64.cc:
* src/x64/deoptimizer-x64.cc:
* src/x64/full-codegen-x64.cc:
* src/x64/lithium-codegen-x64.cc:
* src/x64/regexp-macro-assembler-x64.cc:
* src/x64/stub-cache-x64.cc: Remove a number of assigned but
  unreferenced variables.

* SConstruct (CCTEST_EXTRA_FLAGS): Punt on -Wunused-but-set-variable for
  the test suite.

BUG=1291
TEST=A build and tools/test.py passes.

Review URL: http://codereview.chromium.org/7400023

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8688 ce2b1a6d-e550-0410-aec6-3dcde31c8c00