x86: fix fault_msg nul termination

The fault_msg text is not explictly nul terminated now in startup
assembly. Do so by converting .ascii to .asciz.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>