nptl: Use internal low-level lock type for !IS_IN (libc)

This avoids an ABI hazard (types changing between different modules
of glibc) without introducing linknamespace issues. In particular,
NSS modules now call __lll_lock_wait_private@@GLIBC_PRIVATE to wait
on internal locks (the unlock path is inlined and performs a direct
system call).

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>