Add exec configuration to rust_toolchain::rustc_lib (#856)

Author: UebelAndre

rust/toolchain.bzl

@@ -295,6 +295,7 @@ rust_toolchain = rule(
         ),
         "rustc_lib": attr.label(
             doc = "The libraries used by rustc during compilation.",
+            cfg = "exec",
         ),
         "rustc_srcs": attr.label(
             doc = "The source code of rustc.",