Add AArch64 unit tests

Add unit tests for checking a few tricky instruction sizes. Also remove the old
tests for the instruction sizes, which were clunky and brittle.

Since this is the first set of target-specific unit tests, we need to add some
CMake plumbing. In the future, adding unit tests for a given target will be as
simple as creating a directory with the same name as the target under
unittests/Target. The tests are only run if the target is enabled in
LLVM_TARGETS_TO_BUILD.

Differential Revision: https://reviews.llvm.org/D24548

llvm-svn: 283990