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.

* wtf/Atomics.h:
(WTF::atomicIncrement):
(WTF::atomicDecrement):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@139553 268f45cc-cd09-0410-ab3c-d52691b4dbfc