IDA: Fix ida(64) executable detection.

[alula]

The 7e92f6a commit added a condition that checks the IDA_SDK_VERSION, which is not actually defined for the binexport_shared target (that builds util/idb_export.cc).

https://github.com/google/binexport/blob/main/util/idb_export.cc#L61-L74

I forgot about this being an issue until now, because it has just materialized on a fresh installation of IDA. At one point, I've symlinked ida to ida64 locally to work around that issue 😆 (and I assume authors of previous pull requests did something similar).

Since IdbExporter is also used by the bindiff command line tool, I've figured that the best approach would be implementing executable detection, as I don't know of any other sane way to determine the IDA version (idat doesn't seem to have --version switch or anything...). If there's a better way to approach this, I welcome suggestions.