libstdc++/testsuite: Unpoison 'u' on s390x in names.cc test

libstdc++/testsuite: Unpoison 'u' on s390x in names.cc test

This is the s390 counterpart to r11-7364-gd0453cf5c68b6a, and fixes the
following names.cc failure caused by a use of a poisoned identifier.
If we look at the corresponding upstream header[1] it's clear that the
problematic identifier is 'u'.

In file included from /usr/include/linux/types.h:5,
                 from /usr/include/linux/sched/types.h:5,
                 from /usr/include/bits/sched.h:61,
                 from /usr/include/sched.h:43,
                 from /usr/include/pthread.h:22,
                 from /usr/include/c++/14/s390x-redhat-linux/bits/gthr-default.h:35,
                 from /usr/include/c++/14/s390x-redhat-linux/bits/gthr.h:157,
                 from /usr/include/c++/14/ext/atomicity.h:35,
                 from /usr/include/c++/14/bits/ios_base.h:39,
                 from /usr/include/c++/14/streambuf:43,
                 from /usr/include/c++/14/bits/streambuf_iterator.h:35,
                 from /usr/include/c++/14/iterator:66,
                 from /usr/include/c++/14/s390x-redhat-linux/bits/stdc++.h:54,
                 from /root/rpmbuild/BUILD/gcc-14.3.1-20250617/libstdc++-v3/testsuite/17_intro/names.cc:384:
/usr/include/asm/types.h:24: error: expected unqualified-id before '[' token
/usr/include/asm/types.h:24: error: expected ')' before '[' token
/root/rpmbuild/BUILD/gcc-14.3.1-20250617/libstdc++-v3/testsuite/17_intro/names.cc:101: note: to match this '('
compiler exited with status 1
FAIL: 17_intro/names.cc  -std=gnu++98 (test for excess errors)
Excess errors:
/usr/include/asm/types.h:24: error: expected unqualified-id before '[' token
/usr/include/asm/types.h:24: error: expected ')' before '[' token

[1]: https://github.com/torvalds/linux/blob/master/arch/s390/include/uapi/asm/types.h

libstdc++-v3/ChangeLog:

	* testsuite/17_intro/names.cc: Undefine 'u' on s390*-linux.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>