builtins: make ARM compilation with GAS work again

The LLVM IAS seems to accept wide instructions for add and sub in ARM mode even
though it is not permitted.  This uses a macro to ensure that the wide modifier
is only applied when building in THUMB mode.

This repairs building with GCC/GAS in ARM mode.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@214046 91177308-0d34-0410-b5e6-96231b3b80d8