Add buildbot with --enable-big-digits=15 configure option?

[skirpichev]

It seems only default value (30) is tested both in the CPython CI and with buildbots.

Here some arguments to keep this configuration:

• Drop support for 15-bit PyLong digits? cpython#89732 (comment)
• Test 15-bit digit case for PyLong's in CI? cpython#138336 (comment)

See python/cpython#138336.

[gpshead]

My bot is covering this now.

[skirpichev]

Unfortunately, it seems that the only way to find this bot is search over this repo. Though, this is not specific just for that option.

[StanFromIreland]

Greg can add it to the host string or one can add a tag (e.g. factory_tags = ['15-bit-digits']) to the SlowNonDebugUnixBuild15BitDigits factory.

[skirpichev]

Tag looks better, if it searchable.