htl: Move cleanup handling to non-private libc-lock

This adds sysdeps/htl/libc-lock.h which augments sysdeps/mach/libc-lock.h with
the htl-aware cleanup handling. Otherwise inclusion of libc-lock.h
without libc-lockP.h would keep only the mach-aware handling.

This also fixes cleanup getting called when the binary is
statically-linked without libpthread.

* sysdeps/htl/libc-lockP.h (__libc_cleanup_region_start,
__libc_cleanup_end, __libc_cleanup_region_end,
__pthread_get_cleanup_stack): Move to...
* sysdeps/htl/libc-lock.h: ... new file.
(__libc_cleanup_region_start): Always set handler and arg.
(__libc_cleanup_end): Always call the cleanup handler.
(__libc_cleanup_push, __libc_cleanup_pop): New macros.