Target specific ABI tweaks

[Paul Brook]

I'm trying to implement the arm C++ EABI. Some of these require changes to 
libstdc++. In particular guard variables and array cookies have a different 
format. We also have several additional EABI specified routines that need 
adding to libsupc++.

What would be the preferred way to make these changes? I can't see any 
existing mechanisms for describing target differences from the generic C++ 
abi.

Paul