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

fusl/src/setjmp/aarch64/longjmp.s

+.global _longjmp
+.global longjmp
+.type _longjmp,%function
+.type longjmp,%function
+_longjmp:
+longjmp:
+        // IHI0055B_aapcs64.pdf 5.1.1, 5.1.2 callee saved registers
+        ldp x19, x20, [x0,#0]
+        ldp x21, x22, [x0,#16]
+        ldp x23, x24, [x0,#32]
+        ldp x25, x26, [x0,#48]
+        ldp x27, x28, [x0,#64]
+        ldp x29, x30, [x0,#80]
+        ldr x2, [x0,#104]
+        mov sp, x2
+        ldp d8 , d9, [x0,#112]
+        ldp d10, d11, [x0,#128]
+        ldp d12, d13, [x0,#144]
+        ldp d14, d15, [x0,#160]
+
+        mov x0, x1
+        cbnz x1, 1f
+        mov x0, #1
+1:      br x30