This PR enables the option `-fpatchable-function-entry` for SystemZ. It
utilizes existing common code and just adds the emission of nops after
the function label in the backend.
SystemZ provides multiple nop options of varying length, making the
semantics of this option somewhat ambiguous. In order to align with what
`gcc` does with that same option, we#re choosing `nopr` as the
canoonical nop for this purpose.
For test, this adapts an existing test file from aarch64.
