This fix was motivated by a code generation bug in g++ on arm, but it
seems a good idea generally to disable these unit tests if we're not
compiling with optimisations, since the code tested is only intended
to be used as inlined functions, and while it's possible to compile
without optimisations, the tests become somewhat half-hearted since
there are workarounds when there's no inlining (such as for
instrinsics requiring immediate values), so the tests would partly
test code that wont be used anyway.

BUG=aomedia:377

Change-Id: I9a0515c96a7ed2f4636820dfc03fbb92323ca8ee