For compiletests, use the linker if present.

When running x.py test tests/ui with a custom linker script, we found
that 204 tests actually invoke the default linker (/usr/bin/ld) instead
of the linker script. Depending on the system linker, this will cause
those tests to fail. Many of the failing tests (196) use
"// aux-build:..." directives. One such example is
tests/ui/annotate-snippet/multispan.rs.

In cases where there were failures, we ran with -v and confirmed that we
saw "linker: Some(<path deleted>)" when the compiletest configuration
was dumped. Nevertheless, the rustc command lines for the failing tests
did not contain -Clinker=XXX, resulting in errors like the following:

= note: /usr/bin/ld: /path/to/libunwind.a(UnwindLevel1-gcc-ext.o): unrecognized relocation (0x2a) in section `.text._Unwind_GetDataRelBase'
        /usr/bin/ld: final link failed: Bad value
	collect2: error: ld returned 1 exit status

src/tools/compiletest/src/runtest.rs

@@ -1897,6 +1897,10 @@ impl<'test> TestCx<'test> {
         // Use a single thread for efficiency and a deterministic error message order
         rustc.arg("-Zthreads=1");
 
+        if let Some(ref linker) = self.config.linker {
+            rustc.arg(format!("-Clinker={}", linker));
+        }
+
         // Optionally prevent default --target if specified in test compile-flags.
         let custom_target = self.props.compile_flags.iter().any(|x| x.starts_with("--target"));