Fix typo in definition of `build-art-executable`.

Uses of `LOCAL_MODULES` need two `$` signs within that Make
function definition, so the variable is properly evaluated,
as `build-art-executable` is expanded twice (because it is
used in calls to the `eval` function).

Test: ART_BUILD_HOST_STATIC=true mmma art
Change-Id: I669d493aa7470cb8cde28b2018c784937cd2d335