Allow subscripting raw pointers

This will be useful for dealing with vectors; regardless of our final solution
for the Index trait.

2016-07-06  Manish Goregaokar  <manish@mozilla.com>

gdb/ChangeLog:
    * rust-lang.c (rust_subscript): Allow subscripting pointers

gdb/testsuite/ChangeLog:
    * simple.rs: Add test for raw pointer subscripting
    * simple.exp: Add test expectations