gccrs: Fix up DejaGnu directives in 'rust/compile/issue-1830_{bytes,str}.rs' test cases [#1838]

'dg-excess-errors' "indicates that the test is expected to fail due to compiler
messages that are not handled by 'dg-error', [etc.]".  It's argument doesn't
specify the message to match but just just a comment.  Here, however, we'd like
to match specific compiler messages, so we should use 'dg-error', etc.  Use the
magic line number zero as the compiler error diagnostic doesn't tell which line
the error occurred on (separate issue to resolve).  This gives us:

    -XFAIL: rust/compile/issue-1830_str.rs (test for excess errors)
    +PASS: rust/compile/issue-1830_bytes.rs  at line 8 (test for errors, line )
    +PASS: rust/compile/issue-1830_bytes.rs (test for excess errors)
    -XFAIL: rust/compile/issue-1830_bytes.rs (test for excess errors)
    +PASS: rust/compile/issue-1830_str.rs  at line 8 (test for errors, line )
    +PASS: rust/compile/issue-1830_str.rs (test for excess errors)

Fix-up for recent GCC/Rust master branch #1838
commit cf9ed4001eec6a22686d19a13f7de8ac94f9cb2a
"testsuite: Add empty string macro test".

	gcc/testsuite/
	* rust/compile/issue-1830_bytes.rs: Fix up DejaGnu directives.
	* rust/compile/issue-1830_str.rs: Likewise.