Set TARGET_ARCHITECTURE=none for all clang toolchains

Summary

This MR workarounds a bug in clang that leads to a segfault in the compiler when compiling with architecture flags like -mno-sse. See core/dune-istl#93 (closed), core/dune-common#132 (closed), #9, and !46 (merged)

Details

The target architecture for detection of optimization flags is explicitly set to none to prevent the Vc config package from adding those flags. This is done for all toolchains involving clang-6 and clang-7 even those with noassert.

Note

This change is necessary to build release 2.6 only. Maybe there is an alternative check to guarantee that Vc package is new enough, instead of having a fixed deactivation.