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

fusl/src/exit/exit.c

+#include <stdlib.h>
+#include <stdint.h>
+#include "libc.h"
+
+static void dummy()
+{
+}
+
+/* atexit.c and __stdio_exit.c override these. the latter is linked
+ * as a consequence of linking either __toread.c or __towrite.c. */
+weak_alias(dummy, __funcs_on_exit);
+weak_alias(dummy, __stdio_exit);
+weak_alias(dummy, _fini);
+
+__attribute__((__weak__, __visibility__("hidden")))
+extern void (*const __fini_array_start)(void), (*const __fini_array_end)(void);
+
+static void libc_exit_fini(void)
+{
+        uintptr_t a = (uintptr_t)&__fini_array_end;
+        for (; a>(uintptr_t)&__fini_array_start; a-=sizeof(void(*)()))
+                (*(void (**)())(a-sizeof(void(*)())))();
+        _fini();
+}
+
+weak_alias(libc_exit_fini, __libc_exit_fini);
+
+_Noreturn void exit(int code)
+{
+        __funcs_on_exit();
+        __libc_exit_fini();
+        __stdio_exit();
+        _Exit(code);
+}