Prepare for inclusion into libstd

This applies the same change as gimli-rs/gimli#503 to this crate.

Cargo.toml

@@ -22,6 +22,12 @@ smallvec = { version = "1", default-features = false, optional = true }
 rustc-demangle = { version = "0.1", optional = true }
 cpp_demangle = { version = "0.2", default-features = false, optional = true }
 
+# Internal feature, only used when building as part of libstd, not part of the
+# stable interface of this crate.
+core = { version = '1.0.0', optional = true, package = 'rustc-std-workspace-core' }
+alloc = { version = '1.0.0', optional = true, package = 'rustc-std-workspace-alloc' }
+compiler_builtins = { version = '0.1.2', optional = true }
+
 [dev-dependencies]
 memmap = "0.7"
 clap = "2"
@@ -41,6 +47,10 @@ default = ["rustc-demangle", "cpp_demangle", "std-object", "fallible-iterator",
 std = ["gimli/std"]
 std-object = ["std", "object", "object/std", "object/compression", "gimli/endian-reader"]
 
+# Internal feature, only used when building as part of libstd, not part of the
+# stable interface of this crate.
+rustc-dep-of-std = ['core', 'alloc', 'compiler_builtins', 'gimli/rustc-dep-of-std']
+
 [[test]]
 name = "output_equivalence"
 harness = false