test(build-std): make mock-std closer to real world

`test_std_on_unsupported_target` never really succeed to build those targets, due to

* local rustup may not have `{aarch64,x86_64}-unknown-none` installed.
* `core` and `compiler-builtins` mock crate are not `no_std` nor `no_core`
* the dummy `main.rs` uses `println!` and is not `no_std`.

This commit make it compile, if you have those targets installed.

tests/testsuite/mock-std/library/compiler_builtins/src/lib.rs

@@ -1 +1 @@
-// intentionally blank
+#![no_std]

tests/testsuite/mock-std/library/core/src/lib.rs

@@ -1,3 +1,4 @@
+#![no_std]
 #![feature(staged_api)]
 #![stable(since = "1.0.0", feature = "dummy")]
 

tests/testsuite/standard_lib.rs

@@ -395,12 +395,13 @@ fn test_std_on_unsupported_target() {
 
     let p = project()
         .file(
-            "src/main.rs",
+            "src/lib.rs",
             r#"
-        fn main() {
-            println!("hello");
-        }
-        "#,
+                #![no_std]
+                pub fn foo() {
+                    assert_eq!(u8::MIN, 0);
+                }
+            "#,
         )
         .build();