[ELF] Rename ARM Thunks in anticipation of Range Thunks

    
The existing names for the ARM and Thumb Thunks highlight their current
use as interworking Thunks. These Thunks can also be used for range
extension Thunks where there is no state change. This change makes the name
more generic so it is suitable for range extension.

Differential Revision: https://reviews.llvm.org/D31605

llvm-svn: 299418