Autodetect the type of allocator crate used

Annotate the allocator crates (allocator_system, allocator_jemalloc) by
the type of allocator they are. If one is requested as an exe allocator,
detect its type by the flags.

This has the effect that using this (de jure wrong) configuration in the
target spec works instead of producing a really unhelpful and arcane
linker error:

"exe-allocation-crate": "alloc_system"

Fixes #43524.

src/liballoc_system/lib.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![no_std]
+#![allow(unused_attributes)]
 #![deny(warnings)]
 #![unstable(feature = "alloc_system",
             reason = "this library is unlikely to be stabilized in its current \
@@ -19,7 +20,9 @@
 #![feature(alloc)]
 #![feature(core_intrinsics)]
 #![feature(staged_api)]
+#![feature(rustc_attrs)]
 #![cfg_attr(any(unix, target_os = "redox"), feature(libc))]
+#![rustc_alloc_kind = "lib"]
 
 // The minimum alignment guaranteed by the architecture. This value is used to
 // add fast paths for low alignment values. In practice, the alignment is a