libstdc++: Replace stacktrace effective target with feature test

libstdc++: Replace stacktrace effective target with feature test

Rmove the dejagnu code for checking whether std::stacktrace is supported
and just use the new dg-require-cpp-feature-test directive to check for
__cpp_lib_stacktrace instead.

libstdc++-v3/ChangeLog:

	* testsuite/19_diagnostics/stacktrace/current.cc: Check for
	__cpp_lib_stacktrace instead of check for stacktrace ET.
	* testsuite/19_diagnostics/stacktrace/entry.cc: Likewise.
	* testsuite/19_diagnostics/stacktrace/hash.cc: Likewise.
	* testsuite/19_diagnostics/stacktrace/output.cc: Likewise.
	* testsuite/19_diagnostics/stacktrace/stacktrace.cc: Likewise.
	* testsuite/19_diagnostics/stacktrace/synopsis.cc: Likewise.
	* testsuite/19_diagnostics/stacktrace/version.cc: Likewise.
	* testsuite/23_containers/vector/debug/assign4_backtrace_neg.cc:
	Likewise.
	* testsuite/lib/libstdc++.exp (check_effective_target_stacktrace):
	Remove.