libstdc++: Fix test failure on AIX

libstdc++: Fix test failure on AIX

This fixes a test failure due to a non-reserved name in an AIX system
header (included via <pthread.h>). That name clashes with one of the
names we check our own headers for, so skip checking that name on AIX.

libstdc++-v3/ChangeLog:

	* testsuite/17_intro/names.cc (func): Undef on AIX.