introduce tools/update_cpu_features.zig

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.

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