s390x: mark %r14 as a link register
s390x: mark %r14 as a link register

Even though there is no fixed link register, %r14 is always used as
such.  With this change, I can finally build the s390x glibc CFG.

The long-term solution to handle unusual code is to introduce
SimStateHistory.jump_lr field, whose value can be computed by
SimEngineVEX._handle_irsb based on the last machine instruction
in the IRSB, then set by SimSuccessors._add_successor, and, when
non-None, used by SimSuccessorts._manage_callstack instead of
state.regs._lr.

angr/archinfo