[PATCH]: S390: Fix handling of needles crossing a page in strstr z15 ifunc-variant. [BZ #25226]

[Stefan Liebler]

Hi,

if the specified needle crosses a page-boundary, the s390-z15 ifunc 
variant of strstr truncates the needle which results in invalid results.

This is fixed by loading the needle beyond the page boundary to v18 
instead of v16. The bug is sometimes observable in test-strstr.c in 
check1() and check2() as the haystack and needle is stored on stack. 
Thus the needle can be on a page boundary.

check2 is now extended to test haystack / needles located on stack, at 
end of page and on two pages.

As soon as committed I'll backport it to glibc 2.30 release branch.

Bye
Stefan
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