Pass tune-cpu to LLVM

I think this is how it should work...

src/test/codegen/tune-cpu-on-functions.rs

@@ -0,0 +1,21 @@
+// This test makes sure that functions get annotated with the proper
+// "tune-cpu" attribute in LLVM.
+
+// no-prefer-dynamic
+// ignore-tidy-linelength
+// compile-flags: -C no-prepopulate-passes -C panic=abort -C linker-plugin-lto -Cpasses=name-anon-globals -Z tune-cpu=generic
+
+#![crate_type = "staticlib"]
+
+// CHECK-LABEL: define {{.*}} @exported() {{.*}} #0
+#[no_mangle]
+pub extern fn exported() {
+    not_exported();
+}
+
+// CHECK-LABEL: ; tune_cpu_on_functions::not_exported
+// CHECK-NEXT: ; Function Attrs:
+// CHECK-NEXT: define {{.*}}() {{.*}} #0
+fn not_exported() {}
+
+// CHECK: attributes #0 = {{.*}} "tune-cpu"="{{.*}}"