chore: move compiler defaults from `nim.cfg` to `config.nims`

[ZoomRmc]

At this moment, in some cases setting the default gc/mm setting for the compiler results in a compilation error.
The workaround for this is moving the setting to the nim.cfg file. Having an additional configuration file is not great, so as soon as the fix for the bug lands in stable the setting should be moved back to the config.nims and nim.cfg should be removed.

This is a PR that sould have fixed the issue: nim-lang/Nim#19455. Not sure why we're still hitting it.

[dlesnoff]

Not sure why we're still hitting it.

The compilation error was obtained with Nim 1.6.12 from the post of @vil02 that you linked.
Is it still hit on Nim-devel? If so, please open an issue in the Nim repository and link it here.