gdb: replace `get_exec_file (0)` calls with `current_program_space->exec_filename ()`

Calls of `get_exec_file (0)` are equivalent to just getting the exec
filename from the current program space.  I'm looking to remove
`get_exec_file`, so replace these uses with
`current_program_space->exec_filename ()`.

Remove the `err` parameter of `get_exec_wrapper` since all the calls
that remain pass 1, meaning to error out if no executable is specified.

Change-Id: I7729ea4c7f03dbb046211cc5aa3858ab3a551965
Approved-By: Tom Tromey <tom@tromey.com>