Cherry-pick fprofile-generate-atomic from google/gcc-4_9

Cherry-pick fprofile-generate-atomic from google/gcc-4_9

	Cherry picked (and modified) from google-4_7 branch
	2012-12-26  Rong Xu  <xur@google.com>
	* common.opt (fprofile-update): Add new flag.
	* coretypes.h: Define enum profile_update.
	* doc/invoke.texi: Document -fprofile-update.
	* gcov-io.h: Declare GCOV_TYPE_ATOMIC_FETCH_ADD and
	GCOV_TYPE_ATOMIC_FETCH_ADD_FN.
	* tree-profile.c (gimple_init_edge_profiler): Generate
	also atomic profiler update.
	(gimple_gen_edge_profiler): Likewise.
	* g++.dg/gcov/gcov-threads-1.C: New test.

From-SVN: r239323