ARM: remove unnecessary .arm directive

It was redundant (since ARM mode is the default) and misleading since
(e.g.) Cortex-A15 would not satisfy the #ifdef but would be in ARM
mode regardless.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@200043 91177308-0d34-0410-b5e6-96231b3b80d8