toolchain: xtensa: enable C11 features with xt-clang

If XCC toolchain is clang based, enabled C11 features by
defining TOOLCHAIN_HAS_C_GENERIC and TOOLCHAIN_HAS_C_AUTO_TYPE.
Mirror the definitions in include/zephyr/toolchain/llvm.h

This is required to build Zephyr with features requiring
tagged arguments like LOG_MIPI_SYST_USE_CATALOG.

Suggested-by: Daniel Leung <daniel.leung@intel.com>
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>