Use clang pre-processors for clang build and allow for other pre-processors in general.

Currently, only 'cpp' is used for various pre-processor commands during build, even for clang. CMAKE_CPP is set in make/cpp2asm_support.cmake.

This has the side effect that when trying to set global flags for only clang, this is currently not possible, because the build will fail when running cpp (g++) as pre-processor.

Some parts in the build rely on g++ being the processor, for example the template2expect functions in suite/tests/CMakeLists.txt seem to rely on properties of the g++ pre-processor (fails with clang++).

Also clang++ doesn't take .asm files as input, unless the -xc++ option is given.

This should get sorted out and clang++ should be run as pre-processor when building with clang(++).