Add vecwid256 microarchitecture option to enable -mprefer-vector-width=256 (!965)

cmake/GaudiBuildFlags.cmake

@@ -240,7 +240,11 @@ if(NOT GAUDI_FLAGS_SET EQUAL GAUDI_FLAGS_OPTIONS)
       if(_arch_opt STREQUAL "native")
         message(FATAL_ERROR "you must use 'native-${BINARY_TAG_OS}-${BINARY_TAG_COMP}-${BINARY_TAG_TYPE}'' instead of '${BINARY_TAG_ARCH}+native-${BINARY_TAG_OS}-${BINARY_TAG_COMP}-${BINARY_TAG_TYPE}'")
       endif()
-      set(arch_opts "${arch_opts} -m${_arch_opt}")
+      if(_arch_opt STREQUAL "vecwid256")
+        set(arch_opts "${arch_opts} -mprefer-vector-width=256")
+      else()
+        set(arch_opts "${arch_opts} -m${_arch_opt}")
+      endif()
     endforeach()
     message(STATUS "Arch Flags:       ${arch_opts}")
     # Common compilation flags