libstdc++: Fix backwards logic in 17_intro/names.cc test [PR 97088]

libstdc++: Fix backwards logic in 17_intro/names.cc test [PR 97088]

I meant to undef the names that clash with newlib headers for newlib,
but I only undef'd them for non-newlib targets. This means they still
cause errors for newlib, and aren't tested for other targets.

This fixes the test to check those names for non-newlib targets, and to
undef them to avoid errors for newlib.

libstdc++-v3/ChangeLog:

	PR libstdc++/97088
	* testsuite/17_intro/names.cc: Fix #if condition for names used
	by newlib headers.