Fix ASSERT_VALID_CODE_POINTER for our ARM builds

We want to allow intermixing thumb and ARM for all builds, not only Android.
Modified the macro to do a thumb-compatible null pointer check.

This also works around a miscompilation on QNX where the compiler appeared to
make incorrect assumptions about the address of functions we are taking.

Change-Id: Ib8fc400178e0c2621bde2ca94b3f94041591e19a
Reviewed-by: Lars Knoll <lars.knoll@digia.com>