[Xtensa 1/10] Recognize Xtensa in triple parsing code

I'm sharing initial set of patches that adds LLVM backend for Xtensa architecture.
It is based on this LLVM fork https://github.com/espressif/llvm-xtensa.
I prepared patches by similar way like it was already successfully done for RISCV, i.e. incrementally add an initial MC layer for Xtensa by small chunks which could be reviewable.

Differential Revision: https://reviews.llvm.org/D64826