libstdc++: Fix test that fails due to name clash with old glibc [PR107979]

libstdc++: Fix test that fails due to name clash with old glibc [PR107979]

This test was recently modified to check that the library doesn't use
__unused anywhere, because that's a macro in newlib. But it's also a
macro in old versions of glibc, so the test now fails for those targets.
Disable that check for those targets as well as for newlib.

libstdc++-v3/ChangeLog:

	PR libstdc++/107979
	* testsuite/17_intro/names.cc: Do not check __unused on old
	Glibc versions.