[ARM] Fix typo in subtarget feature list for 7em triple

The list of subtarget features for the 7em triple contains 't2xtpk',
which actually disables that subtarget feature. Correct that to
'+t2xtpk' and test that the instructions enabled by that feature do
actually work.

Differential Revision: http://reviews.llvm.org/D9936

llvm-svn: 238022