partially fix src/test/run-make/coverage-reports when cross-compiling

partially fix src/test/run-make/coverage-reports when cross-compiling

The test does not work on cross-compiled targets because the --target
flag was not passed to rustc inside the test. This commit fixes that by
adding the flag to the invocations.

Note that the test still fails on cross-compiled targets using
remote-test, as remote-test is not capable (yet) of sending back to the
host system the *.profraw file generated by the instrumentation.

Because of that, this is only a partial fix, and the test has been
ignored on cross-compilation.