* init.c (__pthread_initialize_minimal_internal): Check whether

* init.c (__pthread_initialize_minimal_internal): Check whether

	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.