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.
(8bfc4a2ab4)
-S390: Use new __libc_tbegin_retry macro in elision-lock.c.
(53c5c3d5ac)
-S390: Optimize lock-elision by decrementing adapt_count at unlock.
(dd037fb3df)

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.

sysdeps/unix/sysv/linux/s390/htm.h

@@ -119,11 +119,16 @@
 			      ar modification and fp operations.  Some	\
 			      program-interruptions (e.g. a null	\
 			      pointer access) are filtered and the	\
-			      trancsaction will abort.  In this case	\
+			      transaction will abort.  In this case	\
 			      the normal lock path will execute it	\
 			      again and result in a core dump wich does	\
 			      now show at tbegin but the real executed	\
-			      instruction.  */				\
+			      instruction.				\
+			      However it is not guaranteed that this	\
+			      retry operate on the same data and thus	\
+			      may not end in an program-interruption.	\
+			      Note: This could also be used to probe	\
+			      memory for being accessible!  */		\
 			   "2: tbegin 0, 0xFF0E\n\t"			\
 			   /* Branch away in abort case (this is the	\
 			      prefered sequence.  See PoP in chapter 5	\
@@ -152,7 +157,8 @@
      __ret;								\
      })
 
-/* These builtins are correct.  Use them.  */
+/* These builtins are usable in context of glibc lock elision code without any
+   changes.  Use them.  */
 #define __libc_tend()							\
   ({ __asm__ __volatile__ (".machine push\n\t"				\
 			   ".machinemode \"zarch_nohighgprs\"\n\t"	\