[SystemZ][z/OS] Update lowerCall (#68259)

This PR moves some calculation out of `LowerCall` and into
`SystemZXPLINKFrameLowering::processFunctionBeforeFrameFinalized`.
We need to make this change because LowerCall isn't invoked for
functions that don't have function calls, and it is required for some
tooling to work correctly. A function that does not make any calls is
required to allocate 32 bytes for the parameter area required by the
ABI. However, we allocate 64 bytes because this additional space is
utilized by certain tools, like the debugger.

Co-authored-by: Yusra Syeda <yusra.syeda@ibm.com>