llvm: Set warn-stack-size=0 on naked functions.

[alexrp]

This makes LLVM produce a warning if any stack allocations happen.

As suggested by @zeroZshadow after discussion about #21193:

❯ zig4 build-obj naked.zig -target powerpc-freestanding-eabi
warning: naked.zig:1:0: stack frame size (4) exceeds limit (0) in function 'someFunc'

It's not the most ideal way to surface this information, but it's a lot better than silently wrong codegen.

[alexrp]

Just to be clear, this is for stack allocations that LLVM introduces, not us, so #21082 does not help with this. When compiling in debug mode, LLVM's register allocator seems to use a fast but low-quality block-local allocation strategy that results in spills for any branch, which is what happened in #21193.

[andrewrk]

I don't find this to be acceptable. Writing some crap to stderr when the requirements cannot be met is the wrong road to go down. Take it up with the LLVM devs, this is not a good enough API for us.