kbuild: sort hostprogs before passing it to ifneq

The conditional:

  ifneq ($(hostprogs),)

... is evaluated to true if $(hostprogs) does not contain any word but
whitespace characters.

  ifneq ($(strip $(hostprogs)),)

... is a safe way to avoid interpreting whitespace as a non-empty value,
but I'd rather want to use the side-effect of $(sort ...) to do the
equivalent.

$(sort ...) is used in scripts/Makefile.host in order to drop duplication
in $(hostprogs). It is also useful to strip excessive spaces.

Move $(sort ...) before evaluating the ifneq.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>