base: move crt0.h and elf.h to base/internal

Those headers remained unused outside the internal framework. So it is
better to remove them from the public API.

Issue #1832

repos/base-linux/src/test/lx_rmap/main.cc

@@ -14,14 +14,12 @@
 /* Genode includes */
 #include <base/printf.h>
 #include <base/env.h>
-#include <base/crt0.h>
 #include <base/sleep.h>
 #include <base/thread.h>
 #include <util/misc_math.h>
 #include <rm_session/connection.h>
 
 
-
 static void blob() __attribute__((used));
 static void blob()
 {