Link .ARM.exidx with corresponding text section.

The sh_link in the ELF section header of .ARM.exidx should
be filled with the section index of the corresponding text
section.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174372 91177308-0d34-0410-b5e6-96231b3b80d8