x86: fix early boot command line parsing

x86: fix early boot command line parsing

There is no reliable way to encode NUL character as a character so encode
it as a number. Read: http://sourceware.org/binutils/docs/as/Characters.html.
Octal and hex encoding do not work on at least one system (GNU assembler
version 2.22 (x86_64-linux-gnu) using BFD version (GNU Binutils for Debian) 2.22).
Without this fix e.g. no-real-mode option at the end of xen.gz command line
is not detected.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>