Improve the arm/aarch64 mapping symbol handling

Change the mapping symbol handling to handle the case when the mapping
symbols are prefixed with an arbitrary prefix. This isn't strictly standard
compliance, but if all symbols in an object file is prefixed with objcopy
then the prefix will be added to the mapping symbol also. We still want to
treat these symbols as mapping symbols to get the correct address class data.

Differential revision: http://reviews.llvm.org/D12755

llvm-svn: 247400