toolchain-funcs.eclass: New tc-getTARGET_CPP function

toolchain-funcs.eclass: New tc-getTARGET_CPP function

This returns the name of the C preprocessor for the toolchain being
built if CTARGET is defined, or the toolchain being used otherwise. It
is primarily intended to determine characteristics about an existing
toolchain's target as these may differ from what the tuple suggests.

It is not necessary to add the full set of tc-getTARGET_* helper
functions as this is probably the only reason we would ever invoke a
toolchain in the context of CTARGET.