alpha: Use absolute_pointer to define COMMAND_LINE

alpha:allmodconfig fails to build with the following error
when using gcc 11.x.

  arch/alpha/kernel/setup.c: In function 'setup_arch':
  arch/alpha/kernel/setup.c:493:13: error:
	'strcmp' reading 1 or more bytes from a region of size 0

Avoid the problem by declaring COMMAND_LINE as absolute_pointer().

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>