compel, x86_64: Fix sys_clone() definition

Commit 37e4c7bfc264 fixed arm, ppc, x86 (32bit),
while it made wrong definition of x86_64. Fix that.

Also, add commentary to raw fork() implementation.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

compel/arch/x86/plugins/std/syscalls/syscall_64.tbl

@@ -37,7 +37,7 @@ __NR_shutdown			48		sys_shutdown		(int sockfd, int how)
 __NR_bind			49		sys_bind		(int sockfd, const struct sockaddr *addr, int addrlen)
 __NR_setsockopt			54		sys_setsockopt		(int sockfd, int level, int optname, const void *optval, socklen_t optlen)
 __NR_getsockopt			55		sys_getsockopt		(int sockfd, int level, int optname, const void *optval, socklen_t *optlen)
-__NR_clone			56		sys_clone		(unsigned long flags, void *child_stack, void *parent_tid, unsigned long newtls, void *child_tid)
+__NR_clone			56		sys_clone		(unsigned long flags, void *child_stack, void *parent_tid, void *child_tid, unsigned long new_tls)
 __NR_exit			60		sys_exit		(unsigned long error_code)
 __NR_wait4			61		sys_wait4		(int pid, int *status, int options, struct rusage *ru)
 __NR_kill			62		sys_kill		(long pid, int sig)