std: xous: share allocator symbol in tests

When using the testing framework, a second copy of libstd is built and
linked. Use a global symbol for the `DLMALLOC` variable and mark it as
`extern` when building as a test.

This ensures we only have a single allocator even when running tests.

Signed-off-by: Sean Cross <sean@xobs.io>

Author: xobs

library/std/src/sys/xous/alloc.rs

@@ -1,5 +1,7 @@
 use crate::alloc::{GlobalAlloc, Layout, System};
 
+#[cfg_attr(test, linkage = "available_externally")]
+#[export_name = "_ZN16__rust_internals3std3sys4xous5alloc8DLMALLOCE"]
 static mut DLMALLOC: dlmalloc::Dlmalloc = dlmalloc::Dlmalloc::new();
 
 #[stable(feature = "alloc_system_type", since = "1.28.0")]