Add dependencies on the toolchain (clang, etc)

Historically, we've relied on the path to the clang compiler changing in
order to trigger rebuilds instead of direct dependencies on the compiler
itself. That's somewhat more reliable, since the actual $(CLANG) file is
often a script that doesn't actually change during every update.

In Soong, we've added the dependency anyway, so do it here as well. This
makes it easier for my RBE experiments to identify when we intend to use
clang and to send the entire compiler.

Test: treehugger
Test: build a system image with RBE, find that every command that needed
the compiler gets it.
Change-Id: I3838b5d77884394a8c0f7a1ba133142102602084