Add test that __rg_oom doesn't get internalized during LTO

Co-Authored-By: Rémy Rakic <remy.rakic+github@gmail.com>

tests/ui/lto/lto-global-allocator.rs

@@ -0,0 +1,19 @@
+//@ compile-flags: --crate-type cdylib -C lto
+//@ build-pass
+//@ no-prefer-dynamic
+//@ needs-crate-type: cdylib
+
+use std::alloc::{GlobalAlloc, Layout};
+
+struct MyAllocator;
+
+unsafe impl GlobalAlloc for MyAllocator {
+    unsafe fn alloc(&self, _layout: Layout) -> *mut u8 {
+        todo!()
+    }
+
+    unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
+}
+
+#[global_allocator]
+static GLOBAL: MyAllocator = MyAllocator;