[BACKEND] initial llvm codegen for amdgpu (#402)

* added initial llvm codegen for amdgpu

* fixed whitespace

* fixed hsaco gen from ir

* fixed targetmachine for rocm and added GetSource for rocm

* fixed whitespace issues

* changed statement to use less than 100 lines

* added intrinsics for workgroup - rocm

* whitespace - newline error fix

* fixed error msg for workitem-workgroup intrinsics

* added llvm ir dump for rocm codegen

* [ROCM] changed codegen to emit proper amdgpu kernel header

* fixed whitespace error

* fixed whitespace error- 2

* fixed AddFunction to not to use extra arg

1. Changed AddFunctionInternal to not to take extra arg for target type
2. Use Target from CodeGenLLVM to check for AMDGPU target

* fixed whitespaces

* fixed whitespaces 2

* fixed codegen for AMDGPU - now generating valid IR

* fixed codegen depending on code review

* reviewed alignment for amd devices

* added code to dump code object to file

* fixed cpplint errors

* print out IR after pass manager

* added code to dump asm, obj to file and std string

* fixed whitespaces

* Update codegen_amdgpu.cc

* used registry for amdgpu llvm

* Fixed whitespaces

* added code for calling linker

* fixed formatting errors

* added rocm link python interface

* fixed pylint issues and added more body to the function

* added doc string

* added doc string for module

* fixed python code after review, fixed llvm object codegen

* fixed linker to generate code object

* removed dumping to output file and debugging log out

* fixed lint for python code

* added fault check after running linker

* removed print statement in rocm.py

* changed rocm lld linker to raise runtimeerror than emitting error log to stderr

* changed the way linker command line is pass to subprocess.popen

* removed redundant code and reuse tvm utils

* removed commented out code

* removed cloning of unused modules, and put IR into string