htl: Fix cleanup support for IO locking

* sysdeps/htl/stdio-lock.h: New file, registers locking cleanup to htl.
* sysdeps/htl/libc-lockP.h: Include <libc-lock.h>.
(__libc_cleanup_region_start, __libc_cleanup_end,
__libc_cleanup_region_end): Override macros from <libc-lock.h> with
versions which register cleanup to htl.
(__pthread_get_cleanup_stack): Make reference weak for skipping
registration on in the static non-libpthread case.