Bring in WebKit revisions 139553 and 139921.

They are primarily useful for getting rid of some libstdc++-specific
includes which break the build with libc++, and also for simplifying the
code and removing a Darwin-specific code path.

r139553 by ossy@webkit.org:
  Use sync_add_and_fetch instead of gnu_cxx::exchange_and_add
  https://bugs.webkit.org/show_bug.cgi?id=106729

  After r139514 we need atomicIncrement(int64_t volatile*) for all
  platform. Now the GCC implementation of atomicIncrement() is based on
  gnu_cxx::exchange_and_add, which doesn't support int64_t type, but
  sync_add_and_fetch does.

  Reviewed by Benjamin Poulain.

r139921 by benjamin@webkit.org:
  Use GCC's implementation of atomicIncrement/Decrement on Mac
  https://bugs.webkit.org/show_bug.cgi?id=106976

  Reviewed by Filip Pizlo.

  wtf/Atomics.h: GCC and LLVM have builtin for atomic ADD and SUB:
                 sync_add_and_fetch, sync_sub_and_fetch.

  Using them let the compiler just generate the atomic operations inline
  instead of generating a function call to LibC. It also simplify the
  code a bit.

Cherry-picked from qtscript/184fc178f4c753727445694c4830c250f53da588.

Change-Id: Id785e35944682691725947e0f329668d17ff6903
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>