feature-arm-thumb.inc: fix ARM_THUMB_SUFFIX for armv8
The current definition for ARM_THUMB_SUFFIX doesn't consider armv8
and will therefore cause TUNE_PKGARCH to be set incorrectly for
machines which inherit arch-armv8 and don't include aarch64 in
TUNE_FEATURES (ie when building for 32bit ARMv8).
Also fix typo in comments and improve TUNEVALID[thumb] wording.
Signed-off-by: Andre McCurdy <armccurdy@gmail.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
