Add no_mangle to ensure interrupt vectors aren't removed by rustc

The code appears to have been broken in rust-lang/rust#30830. This
commit uses no_mangle as a work-around, since that's special-cased by
the compiler and always kept.

A more proper solution would be to re-export the necessary symbols in
the final crate, which should also ensure that they're kept. This would
either introduce a platform dependency in the main crate, or require a
restructuring of the code so that the kernel binary is produced by the
platform code.

Author: yuriks

src/chips/sam4l/lib.rs

@@ -64,6 +64,7 @@ extern {
 }
 
 #[link_section=".vectors"]
+#[no_mangle] // Ensures that the symbol is kept until the final binary
 pub static BASE_VECTORS: [unsafe extern fn(); 16] = [
     _estack, reset_handler,
     /* NMI */           unhandled_interrupt,
@@ -81,6 +82,7 @@ pub static BASE_VECTORS: [unsafe extern fn(); 16] = [
 ];
 
 #[link_section=".vectors"]
+#[no_mangle] // Ensures that the symbol is kept until the final binary
 pub static IRQS: [unsafe extern fn(); 80] = [generic_isr; 80];
 
 #[no_mangle]