linux(4): Preserve %rcx (return address) like a Linux do

Perhaps, this does not makes much sense as destroyng %rcx declared by
the x86_64 Linux syscall ABI. However,:
a) if we get a signal while we are in the kernel, we should restore
   tf_rcx when preparing machine context for signal handlers.
b) the Linux world is strange, someone can depend on %rcx value
   after syscall, something like go.

Differential Revision:	https://reviews.freebsd.org/D40155
MFC after:		1 month

sys/amd64/linux/linux_sysvec.c

@@ -169,6 +169,8 @@ linux_fetch_syscall_args(struct thread *td)
 
 	/* Restore r10 earlier to avoid doing this multiply times. */
 	frame->tf_r10 = frame->tf_rcx;
+	/* Restore %rcx for machine context. */
+	frame->tf_rcx = frame->tf_rip;
 
 	td->td_retval[0] = 0;
 	return (0);