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

fusl/src/setjmp/aarch64/longjmp.s

Index: fusl/src/setjmp/aarch64/longjmp.s
diff --git a/fusl/src/setjmp/aarch64/longjmp.s b/fusl/src/setjmp/aarch64/longjmp.s
new file mode 100644
index 0000000000000000000000000000000000000000..7c4655fa968c874fb79962f35e0d32cc79edc226
--- /dev/null
+++ b/fusl/src/setjmp/aarch64/longjmp.s
@@ -0,0 +1,24 @@
+.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