[Hexagon] Add hexagon launcher to apps and add to TVM's build system

[csullivan]

Goal: Simplify the build system for running models on hexagon.

This PR moves the hexagon launcher to be a tvm application (in tvm/apps). It also introduces the USE_HEXAGON_LAUNCHER build flag to build the launcher and all dependencies alongside a standard TVM build which consolidates the number of manual build steps from five to one.

(1) libtvm.so (x86_64)

(2) launcher_android (android-aarch64)
(3)  └─►libtvm_runtime.so (android-aarch64)

(4) liblauncher_rpc_skel.so (hexagon)
(5)  └─►libtvm_runtime.a (hexagon)

Also added a section on Compilation with TVM to the launcher readme.

[csullivan]

@kparzysz-quic

[kparzysz-quic]

This is great!

I just started testing it, and the first thing I saw was a bunch of warnings from the Android NDK cmake:

ndk-bundle/build/cmake/android.toolchain.cmake:449 (message):
  An old version of CMake is being used that cannot automatically detect
  compiler attributes.  Compiler identification is being bypassed.  Some
  values may be wrong or missing.  Update to CMake 3.19 or newer to use
  CMake's built-in compiler identification.

I'm using cmake 3.17, the NDK bundle version in my SDK is 22.1.7171670. These warning are most likely safe to ignore, but maybe there could be a note about it somewhere? Your call, I'm fine either way.

[kparzysz-quic]

Thanks!