libstdc++: eh_globals: gthreads: reset _S_init before deleting key

libstdc++: eh_globals: gthreads: reset _S_init before deleting key

Clear __eh_globals_init's _S_init in the dtor before deleting the
gthread key.

This ensures that, in case any code involved in deleting the key
interacts with eh_globals, the key that is being deleted won't be
used, and the non-thread-specific eh_globals fallback will.

for  libstdc++-v3/ChangeLog

	* libsupc++/eh_globals.cc [!_GLIBCXX_HAVE_TLS]
	(__eh_globals_init::~__eh_globals_init): Clear _S_init first.

(cherry picked from commit a33dda016e5acf9c6325ce8a72a1b0238130374e)