[ELF] Add terminating sentinel .ARM.exidx table entry

    
The .ARM.exidx table has an entry for each function with the first entry
giving the start address of the function, the table is sorted in ascending
order of function address. Given a PC value, the unwinder will search the
table for the entry that contains the PC value.
    
If the table entry happens to be the last, the range of the addresses that
the final unwinding table describes will extend to the end of the address
space. To prevent an incorrect address outside the address range of the
program matching the last entry we follow ld.bfd's example and add a
sentinel EXIDX_CANTUNWIND entry at the end of the table. This gives the
final real table entry an upper bound.
    
In addition the llvm libunwind unwinder currently depends on the presence
of a sentinel entry (PR31091).

Differential revision: https://reviews.llvm.org/D26977

llvm-svn: 287869