lib: c: re-organise into separate static modules, plus a single shared library

lib/libc/malloc/CMakeLists.txt

@@ -1,4 +1,4 @@
-set(source_dirs stdlib)
+set(source_dirs stdlib string)
 
 file(GLOB sources *.c *.h)
 

lib/libc/malloc/string/strdup.c

@@ -0,0 +1,14 @@
+#include <stdlib.h>
+#include <string.h>
+
+char *strdup(char *s)
+{
+	size_t len = strlen(s);
+	char *out = malloc(len + 1);
+	if (!out) {
+		return NULL;
+	}
+
+	memcpy(out, s, len + 1);
+	return out;
+}