S390: Use C11-like atomics instead of plain memory accesses in lock elision code.

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.

See also x86 commit ca6e601a:
"Use C11-like atomics instead of plain memory accesses in x86 lock elision."

ChangeLog:

	* sysdeps/unix/sysv/linux/s390/elision-lock.c
	(__lll_lock_elision): Use atomics to load / store adapt_count.
	* sysdeps/unix/sysv/linux/s390/elision-trylock.c
	(__lll_trylock_elision): Likewise.