arm_acle.h: Small cleanup

Since __SIZEOF_LONG_LONG__ is always defined as 8 on ARM targets,
there's no point in checking this. NFC.

Patch by Moritz Roth.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215697 91177308-0d34-0410-b5e6-96231b3b80d8