Remove auto-generated executable list

Keep a list of the useful executables, and use the -host and -target
phony targets to depend on them for build-art-host and build-art-target.

Relanding I8ff4da32a03c219924be7990963e0c3ba90ef316 with fixes to build
the debug versions of executables, which are needed by the tests.

Change-Id: Id941cf5d1a638763cf4cf2bc0412b0782ae21694