* [RUNTIME] Initial implementation of Hexagon runtime support

This is only the TVM runtime. The FastRPC libraries, simulator driver,
etc. will be provided in subsequent commits.

* Fix pylint complaints

* Fix some more pylint complaints

* Add link to the Hexagon SDK website

* Extract VTCM marker into a common variable

* Implement device->device memory copy

* Disable unsigned PDs by default

* Ensure that --hvx_length is present in sim_args if HVX is enabled

* Remove the line about clang from README.md

Apparently things work with libstdc++.

* Mention to set USE_RPC=OFF when building libtvm_runtime.so for Hexagon

* Remember to use codegen_hvx in validate_hvx_length

* Add a line about minimum version of LLVM