gdbserver: handle all eval_result_type values in tracepoint.cc

It was pointed out[1] that after this commit:

  commit 3812b38d
  Date:   Thu Oct 20 11:14:33 2022 +0100

      gdbserver: allow agent expressions to fail with invalid memory access

Now that agent expressions might fail with the error
expr_eval_invalid_memory_access, we might overflow the
eval_result_names array in tracepoint.cc.  This is because the
eval_result_names array does not include a string for either
expr_eval_invalid_goto or expr_eval_invalid_memory_access.

I don't know if having expr_eval_invalid_goto missing is also a
problem, but it feels like eval_result_names should just include a
string for every possible error.

I could just add two more strings into the array, but I figure that a
more robust solution will be to move all of the error types, and their
associated strings, into a new ax-result-types.def file, and to then
include this file in both ax.h and tracepoint.cc in order to build
the enum eval_result_type and the eval_result_names string array.
Doing this means it is impossible to have a missing error string in
the future.

[1] https://inbox.sourceware.org/gdb-patches/01059f8a-0e59-55b5-f530-190c26df5ba3@palves.net/



Approved-By: Pedro Alves <pedro@palves.net>