Remove unused _Unwind_Stop_Fn when building LIBCXXABI_ARM_EHABI.

This was added upstream but is only used when not building for ARM EHABI.

Change-Id: I02d1836bacdb9bebcf4a960f5d2b96f2b8451c01
Merge pull request #89 from awong-dev/stopfn

Remove unused _Unwind_Stop_Fn when building LIBCXXABI_ARM_EHABI.