LD/testsuite: Add STB_GNU_UNIQUE cross-linker test

Some targets are only really, or at least regularly, regression-tested
in a crossed configuration.  Currently we only have native compiled test
cases for the STB_GNU_UNIQUE feature in the linker test suite.  This is
nice, covering run-time semantics even, but quite often not run at all.
Consequently a regression may remain unnoticed for long.

Add a simple test case then to provide basic linker coverage with no
need for a compiler or a native toolchain.

	ld/
	* testsuite/ld-unique/unique.d: New test.
	* testsuite/ld-unique/unique.exp: Run the new test.  Adjust
	messages for compiled tests.