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.

components/profile/Cargo.toml

@@ -10,7 +10,7 @@ name = "profile"
 path = "lib.rs"
 
 [features]
-unstable = []
+unstable = ["jemalloc-sys"]
 
 [dependencies]
 profile_traits = {path = "../profile_traits"}
@@ -31,3 +31,4 @@ regex = "0.2"
 
 [target.'cfg(not(target_os = "windows"))'.dependencies]
 libc = "0.2"
+jemalloc-sys = {version = "0.1.3", optional = true}