Stop relying on linking details of std’s default allocator

We’ve been bitten before by symbol names changing:
https://github.com/servo/heapsize/pull/46
and upstream is planning to stop using jemalloc by default:
https://github.com/rust-lang/rust/issues/33082#issuecomment-309781465

So use the (relatively) new `#[global_allocator]` attribute
to explicitly select the system allocator on Windows
and jemalloc (now in an external crate) on other platforms.
This choice matches current defaults.

tests/unit/profile/lib.rs

@@ -5,6 +5,7 @@
 extern crate ipc_channel;
 extern crate profile;
 extern crate profile_traits;
+extern crate servo_allocator;
 
 #[cfg(test)]
 mod time;