Add check_effective_target_rdynamic and use it in g++.dg/lto/pr69589_0.C.

Add check_effective_target_rdynamic and use it in g++.dg/lto/pr69589_0.C.

gcc/
	* doc/sourcebuild.texi (Effective-Target Keywords, Other attributes):
	Document rdynamic.

gcc/testsuite/

	* g++.dg/lto/pr69589_0.C: Add dg-require-effective-target for
	rdynamic.  Remove dg-skip-if for targets which don't support -rdynamic.
	* lib/target-supports.exp (check_effective_target_rdynamic):
	New proc.

From-SVN: r246004