rules: cleanup: use '$way_*suf' var instead of open-coded '($3_way_)s'

The changes look like:
    -$1/$2/build/%.$$($3_way_)s : $1/%.c $$(LAX_DEPS_FOLLOW) $$$$($1_$2_HC_DEP)
    +$1/$2/build/%.$$($3_ssuf)  : $1/%.c $$(LAX_DEPS_FOLLOW) $$$$($1_$2_HC_DEP)

This way typos will manifest themselves in all ways,
not only non-vanilla.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>