Use __GTHREAD_MUTEX_INIT_FUNCTION on Tru64 UNIX (PR libstdc++/51296)

	PR libstdc++/51296
	* config/os/osf/ctype_base.h,
	config/os/osf/ctype_configure_char.cc,
	config/os/osf/ctype_inline.h, config/os/osf/error_constants.h:
	Copy from config/os/generic.
	* config/os/osf/os_defines.h: Likewise.
	(_GTHREAD_USE_MUTEX_INIT_FUNC, _GTHREAD_USE_COND_INIT_FUNC):
	Define.
	* configure.host <osf*>: Use os/osf for os_include_dir.

From-SVN: r184108