gdb/testsuite: get real executable in gdb.gdb/index-file.exp

gdb/testsuite: get real executable in gdb.gdb/index-file.exp

Similar to a previous patch, if the gdb executable is in fact a libtool
wrapper, we need to get the path to the real executable to load it in
the top-level gdb.

With this change, the test runs on Cygwin, although I do see two
failures:

    FAIL: gdb.gdb/index-file.exp: debug_names files are identical
    FAIL: gdb.gdb/index-file.exp: debug_str files are identical

Change-Id: Ie06d1ece67e61530e5b664e65b5ef0edccaf6afa
Reviewed-By: Keith Seitz <keiths@redhat.com>