lib/libc/amd64/string/strcspn.S: always return earliest match in 17--32 char case

When matching against a set of 17--32 characters, strcspn() uses two
invocations of PCMPISTRI to match against the first 16 characters
of the set and then the remaining characters. If a match was found in
the first half of the set, the code originally immediately returned
that match. However, it is possible for a match in the second half of
the set to occur earlier in the vector, leading to that match being
overlooked.

Fix the code by checking if there is a match in the second half of the
set and taking the earlier of the two matches.

The correctness of the function has been verified with extended unit
tests and test runs against the glibc test suite.

Approved by: mjg (implicit, via IRC)
MFC after: 1 week
MFC to: stable/14

(cherry picked from commit c91cd7d03a9dee649ba3a1b9b4014df9de111bb8)