Rollup merge of #152432 - fneddy:s390x_packed_stack_attribute, r=nikic

add rustc option -Zpacked-stack

this enables `-Zpacked-stack` just as `-mpacked-stack` in clang and gcc. packed-stack is needed on s390x for kernel development.

For reference: rust-lang/rust#151154 and rust-lang/rust#150766

look at @uweigand s post for full explanation of what this does. Here a wrap-up:

https://github.com/rust-lang/rust/pull/150766#issuecomment-3729074303
> [...]
> packed-stack [...] modifies how the compiler-generated function prolog/epilog code makes use of the 160 byte register save area provided by a caller to the callee [...]  this variant is not actually incompatible with the ABI - packed-stack and regular functions can freely call each other without ABI issues.
> [...]
> combination of -mpacked-stack and -mbackchain [...]  the location in the stack frame where the backchain link ought to be stored is not available. [...] is not supported at all with the default ABI
> [...]
> However, in the special case of also using soft-float, our (implied) soft-float ABI provides a different location for the backchain that is compatible with -mpacked-stack, so that combination should be supported
> [...]