x86, boot: simplify arch/x86/boot/compressed/Makefile

Simplify the arch/x86/boot/compressed/Makefile, by using the new
capability of specifying multiple inputs to a compressor, and the
CONFIG_X86_NEED_RELOCS Kconfig symbol.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>