[libc] Bring libc in-tree

Moving libc from its separate repo into this one, minor resulting build
fixes, and a hacky way to add -I for libc headers in builds.

src/libraries/libc/include/j6libc/cpp.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#ifdef __cplusplus
+#define CPP_CHECK_BEGIN extern "C" {
+#define CPP_CHECK_END   }
+#else
+#define CPP_CHECK_BEGIN
+#define CPP_CHECK_END
+#endif