[Systemz/z/OS] Centralize emitting the call type information

With XPLINK, a no-op with information about the call type is emitted
after each call instruction. Centralizing it has the advantage that it is
easy to document all cases, and it makes it easier to extend it later
(e.g. dynamic stack allocation, 32 bit mode).
Also add a test checking the call types emitted so far.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D119557