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

fusl/src/setjmp/x86_64/setjmp.s

Index: fusl/src/setjmp/x86_64/setjmp.s
diff --git a/fusl/src/setjmp/x86_64/setjmp.s b/fusl/src/setjmp/x86_64/setjmp.s
new file mode 100644
index 0000000000000000000000000000000000000000..98f58b8d6551e391f426fc53c81678a03ac89074
--- /dev/null
+++ b/fusl/src/setjmp/x86_64/setjmp.s
@@ -0,0 +1,22 @@
+/* Copyright 2011-2012 Nicholas J. Kain, licensed under standard MIT license */
+.global __setjmp
+.global _setjmp
+.global setjmp
+.type __setjmp,@function
+.type _setjmp,@function
+.type setjmp,@function
+__setjmp:
+_setjmp:
+setjmp:
+	mov %rbx,(%rdi)         /* rdi is jmp_buf, move registers onto it */
+	mov %rbp,8(%rdi)
+	mov %r12,16(%rdi)
+	mov %r13,24(%rdi)
+	mov %r14,32(%rdi)
+	mov %r15,40(%rdi)
+	lea 8(%rsp),%rdx        /* this is our rsp WITHOUT current ret addr */
+	mov %rdx,48(%rdi)
+	mov (%rsp),%rdx         /* save return addr ptr for new rip */
+	mov %rdx,56(%rdi)
+	xor %rax,%rax           /* always return 0 */
+	ret