Fix the x86_64 vfork implementation.

Change-Id: I599507f5058e6196dc2d5b5653d06d5135dd8ac1
diff --git a/libc/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index cb9af1c..d9a2506 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -83,7 +83,7 @@
 int     setgroups:setgroups32(int, const gid_t*)   arm,x86
 int     setgroups:setgroups(int, const gid_t*)     mips,x86_64
 int     setpgid(pid_t, pid_t)  all
-pid_t   vfork(void)  arm,x86_64
+pid_t   vfork(void)  arm
 int     setregid:setregid32(gid_t, gid_t)  arm,x86
 int     setregid:setregid(gid_t, gid_t)    mips,x86_64
 int     chroot(const char*)  all