S390: Do not clobber r13 with memcpy on 31bit with copies >1MB.

S390: Do not clobber r13 with memcpy on 31bit with copies >1MB.

If the default memcpy variant is called with a length of >1MB on 31bit,
r13 is clobbered as the algorithm is switching to mvcle. The mvcle code
returns without restoring r13. All other cases are restoring r13.

If memcpy is called from outside libc the ifunc resolver will only select
this variant if running on machines older than z10.
Otherwise or if memcpy is called from inside libc, this default memcpy
variant is called.
The testcase timezone/tst-tzset is triggering this issue in some combinations
of gcc versions and optimization levels.

This bug was introduced in commit 04bb21ac93e90d7696bcaf8febe2b2dd2d83585a
and thus is a regression compared to former glibc 2.23 release.

This patch removes the usage of r13 at all. Thus it is not saved and restored.
The base address for execute-instruction is now stored in r5 which is obtained
after r5 is not needed anymore as 256byte block counter.

ChangeLog:

	* sysdeps/s390/s390-32/memcpy.S (memcpy): Eliminate the usage
	of r13 as it is not restored in mvcle case.