rustc: Allow changing the default allocator

This commit is an implementation of [RFC 1183][rfc] which allows swapping out
the default allocator on nightly Rust. No new stable surface area should be
added as a part of this commit.

[rfc]: https://github.com/rust-lang/rfcs/pull/1183

Two new attributes have been added to the compiler:

* `#![needs_allocator]` - this is used by liballoc (and likely only liballoc) to
  indicate that it requires an allocator crate to be in scope.
* `#![allocator]` - this is a indicator that the crate is an allocator which can
  satisfy the `needs_allocator` attribute above.

The ABI of the allocator crate is defined to be a set of symbols that implement
the standard Rust allocation/deallocation functions. The symbols are not
currently checked for exhaustiveness or typechecked. There are also a number of
restrictions on these crates:

* An allocator crate cannot transitively depend on a crate that is flagged as
  needing an allocator (e.g. allocator crates can't depend on liballoc).
* There can only be one explicitly linked allocator in a final image.
* If no allocator is explicitly requested one will be injected on behalf of the
  compiler. Binaries and Rust dylibs will use jemalloc by default where
  available and staticlibs/other dylibs will use the system allocator by
  default.

Two allocators are provided by the distribution by default, `alloc_system` and
`alloc_jemalloc` which operate as advertised.

Closes #27389

src/liballoc/lib.rs

@@ -61,6 +61,7 @@
 #![crate_name = "alloc"]
 #![crate_type = "rlib"]
 #![staged_api]
+#![allow(unused_attributes)]
 #![unstable(feature = "alloc",
             reason = "this library is unlikely to be stabilized in its current \
                       form or name")]
@@ -69,6 +70,7 @@
        html_root_url = "https://doc.rust-lang.org/nightly/",
        test(no_crate_inject))]
 #![no_std]
+#![cfg_attr(not(stage0), needs_allocator)]
 
 #![feature(allocator)]
 #![feature(box_syntax)]
@@ -92,13 +94,13 @@
 #![feature(unsize)]
 #![feature(core_slice_ext)]
 #![feature(core_str_ext)]
+#![cfg_attr(stage0, feature(alloc_system))]
+#![cfg_attr(not(stage0), feature(needs_allocator))]
 
 #![cfg_attr(test, feature(test, rustc_private, box_raw))]
-#![cfg_attr(all(not(feature = "external_funcs"), not(feature = "external_crate")),
-            feature(libc))]
 
-#[cfg(all(not(feature = "external_funcs"), not(feature = "external_crate")))]
-extern crate libc;
+#[cfg(stage0)]
+extern crate alloc_system;
 
 // Allow testing this library