devel/valgrind-devel: finish Makefile reordering, no functional change

Mea culpa, the rest of this re-ordering is due to having missed this line in
a bunch of portlint noise about the patches that will soon be disappearing:

WARN: Makefile: "USES" has to appear earlier.

Move conflicts/bb/options definitions further down, as is also done in the
conflated diff of D25452.

Approved by: zeising (maintainer, implicit)