[mlgo] Skip AOT-compiling a model if a header/object pair is provided

This allows one to cross-compile the header/object for a model in a
setup where the compiler is built on a system that cannot host the AOT
compiler. For example, if arm-hostable clang is desired, while the AOT
Tensorflow compiler can cross-compile to arm, it can't currently run on
arm.

The only alternative in that scenario would be to cross-compile clang
itself, but that gets complicated when trying to run tests after that.

Differential Revision: https://reviews.llvm.org/D99992