gcov: Fix counter update method selection

gcov: Fix counter update method selection

The counter update method selection had some issues.

For PROFILE_UPDATE_ATOMIC, if atomic updates are not supported, then
fallback to single mode, however, use partial atomic updates if
available.  Issue warnings.

For PROFILE_UPDATE_PRFER_ATOMIC, if atomic updates are not supported,
then fallback to single mode, however, use partial atomic updates if
available.  Do not issue warnings.

gcc/ChangeLog:

	* tree-profile.cc (tree_profiling): Do not use atomic operations
	if they are not available.  Try to use at least partial atomic
	updates as a fallback.