runkernel: sort out cargo rigmarole with rustc link arg

.cargo/config

@@ -0,0 +1,2 @@
+[unstable]
+extra-link-arg = true

runkernel/.cargo/config

@@ -1,3 +0,0 @@
-[build]
-rustflags = ["-C", "link-args=-rdynamic"]
-

runkernel/build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("cargo:rustc-link-arg=-rdynamic");
+}