x86: boot/compressed/Makefile: fix "make clean"

The Kbuild variable "targets" is supposed to be
configuration-independent and reflect "all possible targets".  This is
required to make "make clean" work properly.

Therefore, move all manipulation of "targets" as well as custom rules
out of the x86-32 ifdef statement.  Only leave inside the ifdefs the
things that are genuinely configuration-dependent.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>