CodeGen: rename __builtin_arm_sevl to __sevl

ACLE adds the __sevl() extension.  Rename the hint from a custom name to the
ACLE specified name.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207829 91177308-0d34-0410-b5e6-96231b3b80d8