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

fusl/src/thread/aarch64/clone.s

+// __clone(func, stack, flags, arg, ptid, tls, ctid)
+//         x0,   x1,    w2,    x3,  x4,   x5,  x6
+
+// syscall(SYS_clone, flags, stack, ptid, tls, ctid)
+//         x8,        x0,    x1,    x2,   x3,  x4
+
+.global __clone
+.type   __clone,%function
+__clone:
+        // align stack and save func,arg
+        and x1,x1,#-16
+        stp x0,x3,[x1,#-16]!
+
+        // syscall
+        uxtw x0,w2
+        mov x2,x4
+        mov x3,x5
+        mov x4,x6
+        mov x8,#220 // SYS_clone
+        svc #0
+
+        cbz x0,1f
+        // parent
+        ret
+        // child
+1:      ldp x1,x0,[sp],#16
+        blr x1
+        mov x8,#93 // SYS_exit
+        svc #0