Disable gdc execution tests without libphobos

Disable gdc execution tests without libphobos

	gcc/testsuite:
	* lib/target-supports.exp (check_compile): Handle D.
	(check_effective_target_d_runtime): New proc.
	* lib/gdc-dg.exp (gdc-dg-test): Demote link and run tests to
	compile unless d_runtime.
	* gdc.dg/runnable.d: Skip unless d_runtime.
	* gdc.dg/lto/lto.exp: Require d_runtime to run tests.

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

From-SVN: r266438