Remove extern on the pub fn rust_oom lang item in libstd, to match ABI of the declaration in liballoc

Author: SimonSapin

src/libstd/alloc.rs

@@ -128,7 +128,7 @@ fn default_alloc_error_hook(layout: Layout) {
 #[cfg_attr(stage0, lang = "oom")]
 #[cfg_attr(not(stage0), alloc_error_handler)]
 #[unstable(feature = "alloc_internals", issue = "0")]
-pub extern fn rust_oom(layout: Layout) -> ! {
+pub fn rust_oom(layout: Layout) -> ! {
     let hook = HOOK.load(Ordering::SeqCst);
     let hook: fn(Layout) = if hook.is_null() {
         default_alloc_error_hook