Simplify kernel library handling

Remove architecture-specific directories.
Enable Vecmathlib for all architectures.
Simplify logic that handles build flags. Move remaining logic into configure.ac.
Improve clz and popcount kernel functions (code taken from TCE architecture).
Improve vload_half and vload_store kernel functions (code taken from ARM architecture).
Correct error in auto-detecting type sizes

The previous mechanism didn't properly distinguish between different languages. (C and OpenCL C may have different sizes for "long".)