Mark panic_fmt as no_mangle to work around rust-lang/rust#38281

(cherry picked from commit 081105bbb60e1c76f222a6c34baac4eec0c83846)
(cherry picked from commit 1acc1ff5806f2fa101f726644d258eb4e4543540)

Author: phil-opp

src/lib.rs

@@ -83,6 +83,7 @@ extern "C" fn eh_personality() {}
 
 #[cfg(not(test))]
 #[lang = "panic_fmt"]
+#[no_mangle]
 extern "C" fn panic_fmt(fmt: core::fmt::Arguments, file: &'static str, line: u32) -> ! {
     println!("\n\nPANIC in {} at line {}:", file, line);
     println!("    {}", fmt);