Yes, let's be "namby Pamby" with the cache lines storing the hot part of the stack, that sounds like an awesome idea!

I thought Zig was all about maximum performance. Sometimes I just want a little bit of stack memory, which will often already be in L1 cache.

Zig does allow this, that's what GP is saying. You don't actually need to relocate your stack, you can just declare a portion of your stack (i.e. what would otherwise be the next N frames) to be the stack you'll use for recursion, and thereafter use that to recurse in to.