Add -Z profile-emit=<path> for Gcov gcda output.

Adds a -Z flag to control the file path that the Gcov gcda output is
written to during runtime. This flag expects a path and filename, e.g.
-Z profile-emit=gcov/out/lib.gcda.

This works similar to GCC/Clang's -fprofile-dir flag which allows
control over the output path for gcda coverage files.

src/librustc_interface/tests.rs

@@ -557,6 +557,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(plt, Some(true));
     tracked!(print_fuel, Some("abc".to_string()));
     tracked!(profile, true);
+    tracked!(profile_emit, Some(PathBuf::from("abc")));
     tracked!(relro_level, Some(RelroLevel::Full));
     tracked!(report_delayed_bugs, true);
     tracked!(run_dsymutil, false);