classes/recipes: Switch virtual/XXX-gcc to virtual/cross-cc (and c++/binutils)

classes/recipes: Switch virtual/XXX-gcc to virtual/cross-cc (and c++/binutils)

The idea of the base class dependency is to say "yes, I need a C cross compiler"
and this was never meant to be gcc specific. Looking at the codebase, whilst we
code triplets into this, it does overcomplicate things as there are only ever
limited, "target", "sdk" and the class extended versions like mutlilib.

After much thought, we can simplify this to virtual/cross-cc and virtual/nativesdk-cross-cc.

This lets us remove the "gcc" specific element as well as removing the over
complicated triplet usage.

At the same time, change the much less widely used "g++" variant to "c++" for
similar reasons and remove the triplet from virtual/XXX-binutils too.

Backwards compatibility mappings could be left but are just going to confuse
things in future so we'll just require users to update.

This simplification, whilst disruptive for any toolchain focused layers, will
make improved toolchain selection in the future much easier.

Since we no longer have overlapping variables, some code for that can just
be removed. The class extension code does need to start remapping some variables
but not the crosssdk target recipe names.

This patch is in two pieces, this one handles the renaming with the functional
changes separate in a second for easier review even if this breaks bisection.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>