don't force-on c feature when working in rustc workspace

Cargo.toml

@@ -58,7 +58,7 @@ mangled-names = []
 no-lang-items = []
 
 # Only used in the compiler's build system
-rustc-dep-of-std = ['c', 'compiler-builtins', 'core']
+rustc-dep-of-std = ['compiler-builtins', 'core']
 
 [[example]]
 name = "intrinsics"