Use C11-like atomics instead of plain memory accesses in x86 lock elision.

This uses atomic operations to access lock elision metadata that is accessed
concurrently (ie, adapt_count fields).  The size of the data is less than a
word but accessed only with atomic loads and stores; therefore, we add
support for shorter-size atomic load and stores too.

	* include/atomic.h (__atomic_check_size_ls): New.
	(atomic_load_relaxed, atomic_load_acquire, atomic_store_relaxed,
	atomic_store_release): Use it.
	* sysdeps/x86/elide.h (ACCESS_ONCE): Remove.
	(elision_adapt, ELIDE_LOCK): Use atomics.
	* sysdeps/unix/sysv/linux/x86/elision-lock.c (__lll_lock_elision): Use
	atomics and improve code comments.
	* sysdeps/unix/sysv/linux/x86/elision-trylock.c
	(__lll_trylock_elision): Likewise.