Ada PATCH: Fix ada/58239 by linking with xg++, not xgcc

[Eric Botcazou]

> This patch fixes that by introducing GXX_LINK which is GCC_LINK except
> that CXX (e.g. xg++) instead of CC (e.g. xgcc) is invoked.
> 
> Eric, are there other executables that need to be linked with GXX_LINK
> too but aren't triggered yet?

Yes, all not covered executables linking with TOOLS_LIBS since it contains 
libcommon.a which now drags the C++ library.  So the simplest solution is to 
change GCC_LINK (there is one potential problematic case, vxaddr2line, but it 
probably didn't link before so let's forget it for now).

I'll attach a patch to the PR so that the Darwin folks can test it.

-- 
Eric Botcazou