LibC: Add malloc.h with some GNU extension

This adds `reallocarray`, `malloc_usable_size`, `mallinfo`, `mallinfo2`

userspace/libraries/LibC/include/malloc.h

@@ -0,0 +1,54 @@
+#ifndef _MALLOC_H
+#define _MALLOC_H 1
+
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+#define __need_size_t
+#include <stddef.h>
+
+struct mallinfo
+{
+	int arena;
+	int ordblks;
+	int smblks;
+	int hblks;
+	int hblkhd;
+	int usmblks;
+	int fsmblks;
+	int uordblks;
+	int fordblks;
+	int keepcost;
+};
+
+struct mallinfo2
+{
+	size_t arena;
+	size_t ordblks;
+	size_t smblks;
+	size_t hblks;
+	size_t hblkhd;
+	size_t usmblks;
+	size_t fsmblks;
+	size_t uordblks;
+	size_t fordblks;
+	size_t keepcost;
+};
+
+void* malloc(size_t total_size);
+void  free(void* ptr);
+void* realloc(void* ptr, size_t size);
+void* calloc(size_t nmemb, size_t size);
+void* reallocarray(void* ptr, size_t nmemb, size_t size);
+void* aligned_alloc(size_t alignment, size_t size);
+int   posix_memalign(void** memptr, size_t alignment, size_t size);
+
+size_t malloc_usable_size(void* ptr);
+
+struct mallinfo mallinfo(void);
+struct mallinfo2 mallinfo2(void);
+
+__END_DECLS
+
+#endif