testsuite: Disable gdc standard runtime tests if phobos is not built.

The current check_effective_target_d_runtime procedure returns false if
the target is built without any core runtime library for D being
available (--disable-libphobos).  This additional procedure is for
targets where the core runtime library exists, but without the higher
level standard library.

gcc/ChangeLog:

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

gcc/testsuite/ChangeLog:

	* gdc.dg/link.d: Use d_runtime_has_std_library effective target.
	* gdc.dg/runnable.d: Move phobos tests to...
	* gdc.dg/runnable2.d: ...here.  New test.
	* lib/target-supports.exp
	(check_effective_target_d_runtime_has_std_library): New.

libphobos/ChangeLog:

	* testsuite/libphobos.phobos/phobos.exp: Skip if effective target is
	not d_runtime_has_std_library.
	* testsuite/libphobos.phobos_shared/phobos_shared.exp: Likewise.