libstdc++: Fix usage of __GXX_WEAK__ in TM TS support.

libstdc++: Fix usage of __GXX_WEAK__ in TM TS support.

	* include/bits/c++config (_GLIBCXX_USE_WEAK_REF): New.
	(_GLIBCXX_TXN_SAFE, _GLIBCXX_TXN_SAFE_DYN): Use	_GLIBCXX_USE_WEAK_REF
	and move after its definition.
	* config/os/aix/os_defines.h (_GLIBCXX_USE_WEAK_REF): Override.
	* src/c++11/cow-stdexcept.cc: Use _GLIBCXX_USE_WEAK_REF instead of
	__GXX_WEAK__, and only provide transactional clones if
	_GLIBCXX_USE_WEAK_REF is true.  Don't provide stubs of libitm
	functions.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@232539 138bc75d-0d04-0410-961f-82ee72b054a4