Add Xtensa MCTargetDesc stub. Add XtensaTargetMachine and XtensaTargetInfo.
Modify llib/Target/LLVMBuild.txt. Now Xtensa target could be builded as EXPERIMENTAL.
Quickly scanning this patch shows that it still uses the previous license. I think these need to be updated to the new Apache 2.0 license, see any file for the up-to-date file header.