x86_64 vfork implementation

untested; should work.

src/process/x86_64/vfork.s

@@ -0,0 +1,12 @@
+.global __vfork
+.weak vfork
+.type __vfork,@function
+.type vfork,@function
+__vfork:
+vfork:
+	pop %rdx
+	mov $58,%eax
+	syscall
+	push %rdx
+	mov %rax,%rdi
+	jmp __syscall_ret