Fix a small bug in gdb.rust/simple.rs
    
    I noticed that gdb.rust/simple.rs had two local variables named "v".
    This didn't previous cause problems, but with a newer rust compiler
    this resulted in a test failure.  (It should have failed all along, so
    I suppose earlier passes were due to a compiler bug.)
    
    This patch renames the second variable.
    
    2018-08-31  Tom Tromey  <tom@tromey.com>
    
    	* gdb.rust/simple.rs: Rename second variable "v".