gdb: make deprecated_show_value_hack use its ui_file parameter

gdb: make deprecated_show_value_hack use its ui_file parameter

If a setting does not provide a "show" function, we fall back to the
deprecated_show_value_hack function to print the value in a generic
way. The current version ignores the `file` parameter specifying the
output file in favor of always using gdb_stdout.

To make things consistent with how the values are printed using a
regular show command, the proposed change makes the function use the
ui_file parameter it receives (even though do_show_command passes
gdb_stdout).

Co-Authored-By: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I54028ed654a1fa4d955e6e46f979472c8d98ace9
Approved-By: Andrew Burgess <aburgess@redhat.com>