[Xtensa 4/10] Add basic *td files with Xtensa architecture description

[Xtensa 4/10] Add basic *td files with Xtensa architecture description

Add initial Xtensa.td file with target machine description. Add XtensaInstrInfo.td,
currently describe just susbet of Core Instructions like ALU, Processor control,
memory barrier and some move instructions. Add descriptions of the instructions
formats(XtensaInstrInfo.td) and some immediate instruction operands(XtensaOperands.td).
Add General Registers and Special Registers classes.

Differential Revision: https://reviews.llvm.org/D64830