Add rustc-dep-of-std feature

This is how `libc` and `hashbrown` support being dependancies of `libstd`.
This allows `getrandom` to use `core` normally, and use the sysroot `core`
when building `libstd`.

Author: josephlr

Cargo.toml

@@ -21,6 +21,10 @@ members = ["tests/wasm_bindgen"]
 log = { version = "0.4", optional = true }
 cfg-if = "0.1"
 
+# When built as part of libstd
+compiler_builtins = { version = "0.1", optional = true }
+core = { version = "1.0", optional = true, package = "rustc-std-workspace-core" }
+
 [target.'cfg(any(unix, target_os = "redox", target_os = "wasi"))'.dependencies]
 libc = "0.2.60"
 
@@ -30,3 +34,5 @@ stdweb = { version = "0.4.18", optional = true }
 
 [features]
 std = []
+# Unstable feature to support being a libstd dependancy
+rustc-dep-of-std = ["compiler_builtins", "core"]