all: remove last traces of the C runtime

This increases code size on the nrf, but it is a fixed increase and can
hopefully be reduced in the future.
The Makefile gets a lot smaller with this which is a huge advantage
(less build time complexity).

src/runtime/runtime.h

@@ -1,13 +0,0 @@
-
-#pragma once
-
-#include <stdint.h>
-
-typedef struct {
-	uint32_t len;  // TODO: size_t (or, let max string size depend on target/flag)
-	uint8_t  *buf; // points to string buffer itself
-} string_t;
-
-typedef int32_t intgo_t; // may be 64-bit
-
-int main();

src/runtime/runtime.ll

@@ -1,3 +0,0 @@
-source_filename = "runtime/runtime.ll"
-
-; dummy file