[SPIR-V](1/6) Add stub for SPIRV backend

This patch contains enough for lib/Target/SPIRV to compile: a basic
SPIRVTargetMachine and SPIRVTargetInfo.

Differential Revision: https://reviews.llvm.org/D115009



Authors: Aleksandr Bezzubikov, Lewis Crawford, Ilia Diachkov,
Michal Paszkowski, Andrey Tretyakov, Konrad Trifunovic

Co-authored-by: Aleksandr Bezzubikov <zuban32s@gmail.com>
Co-authored-by: Ilia Diachkov <iliya.diyachkov@intel.com>
Co-authored-by: Michal Paszkowski <michal.paszkowski@outlook.com>
Co-authored-by: Andrey Tretyakov <andrey1.tretyakov@intel.com>
Co-authored-by: Konrad Trifunovic <konrad.trifunovic@intel.com>