Add a "fork" of musl as //fusl.

fusl/src/malloc/malloc_usable_size.c

Index: fusl/src/malloc/malloc_usable_size.c
diff --git a/fusl/src/malloc/malloc_usable_size.c b/fusl/src/malloc/malloc_usable_size.c
new file mode 100644
index 0000000000000000000000000000000000000000..8cccd9d8d68b2e4fc6a230a7e9f0537a60c2353a
--- /dev/null
+++ b/fusl/src/malloc/malloc_usable_size.c
@@ -0,0 +1,17 @@
+#include <malloc.h>
+
+void *(*const __realloc_dep)(void *, size_t) = realloc;
+
+struct chunk {
+	size_t psize, csize;
+	struct chunk *next, *prev;
+};
+
+#define OVERHEAD (2*sizeof(size_t))
+#define CHUNK_SIZE(c) ((c)->csize & -2)
+#define MEM_TO_CHUNK(p) (struct chunk *)((char *)(p) - OVERHEAD)
+
+size_t malloc_usable_size(void *p)
+{
+	return CHUNK_SIZE(MEM_TO_CHUNK(p)) - OVERHEAD;
+}