private futexes are available.
* allocatestack.c (allocate_stack): Copy private_futex field from
current thread into the new stack.
* sysdeps/unix/sysv/linux/x86_64/libc-lowlevellock.S: Use private
futexes if they are available.
* sysdeps/unix/sysv/linux/i386/i486/libc-lowlevellock.S: Likewise
* sysdeps/unix/sysv/linux/x86_64/lowlevellock.S: Adjust so that change
in libc-lowlevellock.S allow using private futexes.
* sysdeps/unix/sysv/linux/i386/i486/lowlevellock.S: Likewise.
* sysdeps/unix/sysv/linux/x86_64/lowlevellock.h: Define
FUTEX_PRIVATE_FLAG.
* sysdeps/unix/sysv/linux/i386/lowlevellock.h: Likewise.
* sysdeps/unix/sysv/linux/x86_64/pthread_once.S: Use private futexes
if they are available.
* sysdeps/unix/sysv/linux/i386/pthread_once.S: Likewise.
* sysdeps/x86_64/tcb-offsets.sym: Add PRIVATE_FUTEX.
* sysdeps/i386/tcb-offsets.sym: Likewise.
* sysdeps/x86_64/tls.h (tcbhead_t): Add private_futex field.
* sysdeps/i386/tls.h (tcbhead_t): Likewise.