remove #[no_mangle] from panic handler

A recent change on rustc nightly broke panic handlers that also have
the #[no_mangle] attribute. I don't know whether this is something that
they want to support and if they'll fix that, but either way I don't
think there's a strong reason to keep this, so let's just remove it.

Author: Freax13

src/main.rs

@@ -370,7 +370,6 @@ fn enable_write_protect_bit() {
 }
 
 #[panic_handler]
-#[no_mangle]
 pub fn panic(info: &PanicInfo) -> ! {
     use core::fmt::Write;
     write!(printer::Printer, "{}", info).unwrap();