add the rustbuild feature

which is required to use this in the rust-lang/rust repo. I actually
forgot to add this in rust-lang#97

Also, enable this feature by default because people that depend on/build
this crate using Cargo will want to have this crate marked as
the #![compiler_builtins] crate, which is what the rustbuild feature
does

Author: Jorge Aparicio

Cargo.toml

@@ -23,6 +23,8 @@ compiler-rt = { path = "compiler-rt" }
 [features]
 # Build the missing intrinsics from compiler-rt C source code
 c = []
+default = ["rustbuild"]
+rustbuild = []
 weak = ["rlibc/weak"]
 
 [workspace]