add legacy function valloc

it was already declared in stdlib.h, but not defined anywhere.

src/legacy/valloc.c

@@ -0,0 +1,8 @@
+#define _BSD_SOURCE
+#include <stdlib.h>
+#include <limits.h>
+
+void *valloc(size_t size)
+{
+	return memalign(PAGE_SIZE, size);
+}