Allow x86+arm multilib build.

The explicit dependency of dex2oat on installed file of libart-compiler
isn't necessary: the build sets up the dependency automatically when
dex2oat links liblibart-compiler.
Also the way of the explicit dependency is incorrect if multilib is
involved: a 64-bit dex2oat may depend on 32-bit libart-compiler.

Bug: 27526885
Change-Id: I4a9d72b49f5712695ff5a32b88df5eab9df7c1e4