gccrs: testsuite: Add empty string macro test

Add two new tests with empty string for include_str and include_bytes
macros.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-1830_bytes.rs: New test.
	* rust/compile/issue-1830_str.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>