stage2 LLVM: Pass inline assembly outputs directly when not targeting memory

[topolarity]

This change provides a basic implementation of #2349 for stage2.

There's still quite a lot of work before this logic is as complete as what's in Clang, especially considering that we don't account for any target-specific constraints at all. Before we round things out though, we should probably wait until there's a clearer picture for constraints in Zig's dedicated ASM syntax

As a bonus, this also fixes a bug with how we were handling _ identifiers in stage 2.