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

fusl/src/linux/sync_file_range.c

+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <errno.h>
+#include "syscall.h"
+
+int sync_file_range(int fd, off_t pos, off_t len, unsigned flags)
+{
+#if defined(SYS_sync_file_range2)
+        return syscall(SYS_sync_file_range2, fd, flags,
+                __SYSCALL_LL_E(pos), __SYSCALL_LL_E(len));
+#elif defined(SYS_sync_file_range)
+        return syscall(SYS_sync_file_range, fd,
+                __SYSCALL_LL_O(pos), __SYSCALL_LL_E(len), flags);
+#else
+        return __syscall_ret(-ENOSYS);
+#endif
+}