Hello,

The attached, per comments from RichardE, revises the definition and use 
of REGISTER_SIM_REGNO.  I think I've managed to preserve existing behavour.

Briefly, REGISTER_SIM_REGNO() [Hmm, do I doco `legacy' behavour?] 
returns a cardinal indicating the simulators register number, or 
SIM_REGNO_UNAVAILABLE/-1 when the register isn't there.

To preserve current behavour on existing targets, 
LEGACY_SIM_REGNO_IGNORE is returned by the default 
legacy_register_sim_regno() function.

Once I've done with testing I'll commit it (few days),
Andrew