Allow building on clang without __builtin_assume_aligned

Clang versions up to and including 3.5 don't have
__builtin_assume_aligned.
Define it to a reasonable approximation when building with those.

Change-Id: I223ccfda6e6ae55ce29dda5e4cacc1b162728b2c
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>