clang: move get_clang_arch() functions to the common inc file

clang: move get_clang_arch() functions to the common inc file

These functions are useful outside of just the clang recipe, so move
them to a common .inc file so they can be used by other clang-related
recipes.

Also make the function fail if it doesn't recognise the architecture,
instead of returning the empty string and causing mysterious fails later.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>