x86: unify variable names in arch/x86/boot/compressed/misc_??.c

size reports no change in arch/x86/boot/compressed/vmlinux.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>