libstdc++: Fix experimental/names.cc failure on AIX

This fails due to "u" being used in a system header.

FAIL: experimental/names.cc  -std=gnu++17 (test for excess errors)
Excess errors:
/usr/include/sys/poll.h:77: error: expected unqualified-id before ';' token
/usr/include/sys/poll.h:77: error: expected ')' before ';' token

FAIL: experimental/names.cc  -std=gnu++17 (test for excess errors)
Excess errors:
/usr/include/sys/poll.h:102: error: expected unqualified-id before ';' token
/usr/include/sys/poll.h:102: error: expected ')' before ';' token

libstdc++-v3/ChangeLog:

	* testsuite/17_intro/names.cc [_AIX]: Undefine "u".