Define VISIBILITY_HIDDEN when compiling compiler-rt

This is cherry-picked from rust-lang#37817 which seems to be stalled and currently needs to be rebased anyway.

r? @alexcrichton (who authored this change)

Author: Robin Kruppe

src/libcompiler_builtins/build.rs

@@ -94,6 +94,7 @@ fn main() {
         cfg.flag("-fvisibility=hidden");
         cfg.flag("-fomit-frame-pointer");
         cfg.flag("-ffreestanding");
+        cfg.define("VISIBILITY_HIDDEN", None);
     }
 
     let mut sources = Sources::new();