Implement proper RX-b instruction support on Z

[fjeremic]

As part of #3693 we've put in a fix in a6c0e45 to change the format of the BC instruction which whose first usage was introduced in #3779 to an RX-a instruction. The reason this was done is to work around the issue that we currently do not have a proper instruction class to support RX-b instructions.

The task here is to effectively revert a6c0e45 and implement proper support for RX-b instructions in [1] then make use of the new overload within the linkage class where the (currently) only use of the BC instruction is.

[1] https://github.com/eclipse/omr/blob/051896be071812db8a3b71485b014be26c80e7f4/compiler/z/codegen/S390Instruction.hpp#L4422-L4489

[github-actions]

This issue is stale because it has been open 180 days with no activity. Remove stale label or comment on the issue or it will be closed in 60 days.

[fjeremic]

Placing this on the backlog as it is something we want to address still.