alpha: fix formating of stack content

Since version 4.9, the kernel automatically breaks printk calls into
multiple newlines unless pr_cont is used. Fix the alpha stacktrace code,
so that it prints stack trace in four columns, as it was initially
intended.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org	# v4.9+
Signed-off-by: Matt Turner <mattst88@gmail.com>