[StrTable] Mechanically convert Hexagon builtins to use TableGen (#123460)

This switches them to use the common builtin TableGen emission.

The fancy feature string preprocessor tricks are replaced with a fairly
direct translation into TableGen.

All of the actual definitions were created using a quite hack-y Python
script that was never intended to be productionized. It preserves the
order, spacing, and even comments from the original files. For
posterity, the script used is here:

https://gist.github.com/chandlerc/f53c7d735e33eecf388529bd9a6010df

The original `.def` file appears to be generated by some out-of-tree
`iset.py` script, which because it is out of tree I couldn't update. It
should be very straightforward though to update it to generate a similar
structure as was used to produce the `.td` file.

In addition to helping move towards TableGen for all of the builtins,
these builtins in particular can be *much* more efficiently handled
using TableGen when we start emitting string tables for them because it
allows de-duplicating all of the feature strings.

The commit sha parent at the time the PR was made is
7253c6f and at that commit, the
resulting TableGen file produces a `.inc` file that only differs in
whitespace and the order of the builtins defined.

Author: chandlerc