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

fusl/src/setjmp/aarch64/setjmp.s

Index: fusl/src/setjmp/aarch64/setjmp.s
diff --git a/fusl/src/setjmp/aarch64/setjmp.s b/fusl/src/setjmp/aarch64/setjmp.s
new file mode 100644
index 0000000000000000000000000000000000000000..f49288aa1c619bf38ee42d4a11104ac06a171315
--- /dev/null
+++ b/fusl/src/setjmp/aarch64/setjmp.s
@@ -0,0 +1,24 @@
+.global __setjmp
+.global _setjmp
+.global setjmp
+.type __setjmp,@function
+.type _setjmp,@function
+.type setjmp,@function
+__setjmp:
+_setjmp:
+setjmp:
+	// IHI0055B_aapcs64.pdf 5.1.1, 5.1.2 callee saved registers
+	stp x19, x20, [x0,#0]
+	stp x21, x22, [x0,#16]
+	stp x23, x24, [x0,#32]
+	stp x25, x26, [x0,#48]
+	stp x27, x28, [x0,#64]
+	stp x29, x30, [x0,#80]
+	mov x2, sp
+	str x2, [x0,#104]
+	stp  d8,  d9, [x0,#112]
+	stp d10, d11, [x0,#128]
+	stp d12, d13, [x0,#144]
+	stp d14, d15, [x0,#160]
+	mov x0, #0
+	ret