x86: unify arch/x86/boot/compressed/Makefile(s)

Trivial unification of the two Makefiles.
Tested doing a defconfig build for both 32 and 64 bit and
no build changes occured.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>