Log message: Add COMPILER lines to c++ ports which currently use the default. Adjust some existing COMPILER lines with arch restrictions etc. In the usual case this is now using "COMPILER = base-clang ports-gcc base-gcc" on ports with c++ libraries in WANTLIB. This is basically intended to be a noop on architectures using clang as the system compiler, but help with other architectures where we currently have many ports knocked out due to building with an unsuitable compiler - - some ports require c++11/newer so the GCC version in base that is used on these archirtectures is too old. - some ports have conflicts where an executable is built with one compiler (e.g. gcc from base) but a library dependency is built with a different one (e.g. gcc from ports), resulted in mixing incompatible libraries in the same address space. devel/gmp is intentionally skipped as it's on the path to building gcc - the c++ library there is unused in ports (and not built by default upstream) so intending to disable building gmpcxx in a future commit.