add rustc option -Zpacked-stack

[fneddy]

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

For reference: #151154 and #150766

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

#150766 (comment)

[...]
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
[...]

[rustbot]

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[fneddy]

CC @cuviper @RalfJung @folkertdev @workingjubilee

[fneddy]

I'll add tests for assembly output and for target incompatibility.

[bjorn3]

Maybe panic here given that the frontend should already have emitted an error.