[ELF][ARM] Initial implentation of ARM exceptions support

The .ARM.exidx sections contain a table. Each entry has two fields:
- PREL31 offset to the function the table entry describes
- Action to take, either cantunwind, inline unwind, or PREL31 offset to
  .ARM.extab section

The table entries must be sorted in order of the virtual addresses the
first entry of the table describes. Traditionally this is implemented by
the SHF_LINK_ORDER dependency. Instead of implementing this directly we
sort the table entries post relocation. 

The .ARM.exidx OutputSection is described by the PT_ARM_EXIDX program
header

Differential revision: https://reviews.llvm.org/D25127

llvm-svn: 283730