libstdc++: Add dg-require-cpp-feature-test to test feature test macros

This adds a new dejagnu directive which can be used to make a test
depend on a feature test macro such as __cpp_lib_text_encoding. This is
mroe flexible than writing a new dg-require-xxx for each feature.

libstdc++-v3/ChangeLog:

	* testsuite/lib/dg-options.exp (dg-require-cpp-feature-test):
	New proc.
	* testsuite/lib/libstdc++.exp (check_v3_target_cpp_feature_test):
	New proc.
	* testsuite/std/text_encoding/cons.cc: Use new directive to skip
	the test if the __cpp_lib_text_encoding feature test macro is
	not defined.
	* testsuite/std/text_encoding/requirements.cc: Likewise.