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

fusl/src/setjmp/x86_64/longjmp.s

Index: fusl/src/setjmp/x86_64/longjmp.s
diff --git a/fusl/src/setjmp/x86_64/longjmp.s b/fusl/src/setjmp/x86_64/longjmp.s
new file mode 100644
index 0000000000000000000000000000000000000000..e175a4b9606bba41eccc8972c22244e533718f0a
--- /dev/null
+++ b/fusl/src/setjmp/x86_64/longjmp.s
@@ -0,0 +1,22 @@
+/* Copyright 2011-2012 Nicholas J. Kain, licensed under standard MIT license */
+.global _longjmp
+.global longjmp
+.type _longjmp,@function
+.type longjmp,@function
+_longjmp:
+longjmp:
+	mov %rsi,%rax           /* val will be longjmp return */
+	test %rax,%rax
+	jnz 1f
+	inc %rax                /* if val==0, val=1 per longjmp semantics */
+1:
+	mov (%rdi),%rbx         /* rdi is the jmp_buf, restore regs from it */
+	mov 8(%rdi),%rbp
+	mov 16(%rdi),%r12
+	mov 24(%rdi),%r13
+	mov 32(%rdi),%r14
+	mov 40(%rdi),%r15
+	mov 48(%rdi),%rdx       /* this ends up being the stack pointer */
+	mov %rdx,%rsp
+	mov 56(%rdi),%rdx       /* this is the instruction pointer */
+	jmp *%rdx               /* goto saved address without altering rsp */