configuring binutils/ld/testsuite when there is no g++ compiler

[Alan Lehotsky]

I’m working on a port for an embedded system where there’s only a C compiler, no C++, and would like to disable the linker tests that use g++;
I rummaged around thru various config files, but didn’t find anything helpful.

I do see in ld/config/default.exp that there’s code to go find the g++ if the CXX environment variable isn’t set. But I don’t see anywhere that allows me
to disable all the g++ tests in one place.