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

fusl/src/mman/mremap.c

+#define _GNU_SOURCE
+#include <unistd.h>
+#include <sys/mman.h>
+#include <errno.h>
+#include <stdint.h>
+#include <stdarg.h>
+#include "syscall.h"
+#include "libc.h"
+
+static void dummy(void) { }
+weak_alias(dummy, __vm_wait);
+
+void *__mremap(void *old_addr, size_t old_len, size_t new_len, int flags, ...)
+{
+        va_list ap;
+        void *new_addr = 0;
+
+        if (new_len >= PTRDIFF_MAX) {
+                errno = ENOMEM;
+                return MAP_FAILED;
+        }
+
+        if (flags & MREMAP_FIXED) {
+                __vm_wait();
+                va_start(ap, flags);
+                new_addr = va_arg(ap, void *);
+                va_end(ap);
+        }
+
+        return (void *)syscall(SYS_mremap, old_addr, old_len, new_len, flags, new_addr);
+}
+
+weak_alias(__mremap, mremap);