libstdc++: Test 17_intro/names.cc with -D_FORTIFY_SOURCE=2 [PR116210]

Add a new testcase that repeats 17_intro/names.cc but with
_FORTIFY_SOURCE defined, to find problems in Glibc fortify wrappers like
https://sourceware.org/bugzilla/show_bug.cgi?id=32052 (which is fixed
now).

libstdc++-v3/ChangeLog:

	PR libstdc++/116210
	* testsuite/17_intro/names.cc (sz): Undef for versions of Glibc
	that use it in the fortify wrappers.
	* testsuite/17_intro/names_fortify.cc: New test.