[runtimes] Only depend on builtins for same target

We don't need to have built all the builtins before building the
runtimes for a particular target, only the builtins for that target.
While I'm here, rename the variable that stores the builtins dep to
something less generic than `deps`, to minimize the chances of
accidentally using a variable with the same name from an outer scope.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D139913