[Arm][Unwind][libc++abi] Add _Unwind_ForcedUnwind to EHABI.

_Unwind_ForcedUnwind is not mandated by the EHABI but for compatibilty
reasons adding so the interface to higher layers would be the same.
Dropping EHABI specific _Unwind_Stop_Fn definition since it is not defined by EHABI.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D89570