S390: Adjust lock elision code after review.

This patch adjusts s390 specific lock elision code after review
of the following patches:
-S390: Use own tbegin macro instead of __builtin_tbegin.
(8bfc4a2a)
-S390: Use new __libc_tbegin_retry macro in elision-lock.c.
(53c5c3d5)
-S390: Optimize lock-elision by decrementing adapt_count at unlock.
(dd037fb3)

The futex value is not tested before starting a transaction,
__glibc_likely is used instead of __builtin_expect and comments
are adjusted.

ChangeLog:

	* sysdeps/unix/sysv/linux/s390/htm.h: Adjust comments.
	* sysdeps/unix/sysv/linux/s390/elision-unlock.c: Likewise.
	* sysdeps/unix/sysv/linux/s390/elision-lock.c: Adjust comments.
	(__lll_lock_elision): Do not test futex before starting a
	transaction.  Use __glibc_likely instead of __builtin_expect.
	* sysdeps/unix/sysv/linux/s390/elision-trylock.c: Adjust comments.
	(__lll_trylock_elision): Do not test futex before starting a
	transaction.  Use __glibc_likely instead of __builtin_expect.