Force code generation in assembly generation smoke-tests

In llvm/llvm-project@7b23f413d1 , `.text`
started being suppressed from LLVM assembly in cases where it wasn't
strictly necessary. Currently, the sample functions in these two tests
are frequently decided to be IR-only functions, resulting in no code
generation, so LLVM drops the `.text` directive.

Adding `#[no_mangle]` forces these tests back to their original intent -
assembly code is generated, and so a `.text` directive is generated as
well.

tests/assembly/targets/targets-elf.rs

@@ -676,6 +676,8 @@
 #[lang = "sized"]
 trait Sized {}
 
+// Force linkage to ensure code is actually generated
+#[no_mangle]
 pub fn test() -> u8 {
     42
 }

tests/assembly/targets/targets-macho.rs

@@ -83,6 +83,8 @@
 #[lang = "sized"]
 trait Sized {}
 
+// Force linkage to ensure code is actually generated
+#[no_mangle]
 pub fn test() -> u8 {
     42
 }