tsan: properly instrument unaligned accesses

tsan: properly instrument unaligned accesses

If a memory access is unaligned, emit __tsan_unaligned_read/write
callbacks instead of __tsan_read/write.
Required to change semantics of __tsan_unaligned_read/write to not do the user memory.
But since they were unused (other than through __sanitizer_unaligned_load/store) this is fine.
Fixes long standing issue 17:
https://code.google.com/p/thread-sanitizer/issues/detail?id=17

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227231 91177308-0d34-0410-b5e6-96231b3b80d8