This replaces the previous target cpu features tool, taking advantage of
llvm-tblgen --dump-json instead of trying to use python to parse the .td
files.
This is an initial version that has the basics working, including a
simple feature override system, as well as multi-threaded processing.
Follow-up commits will do clean ups to make the diff of the newly generated
source files against previous versions be as desired.
2733ca91b2