ldconfig: Ignore all GDB extension files

ldconfig already ignores files with the -gdb.py suffix, but GDB also
looks for -gdb.gdb and -gdb.scm files. These aren't as widely used, but
libguile at least comes with a -gdb.scm file.

Rename is_gdb_python_file to is_gdb_extension_file, and make it
recognise all three types of GDB extension.

Signed-off-by: Adam Sampson <ats@offog.org>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>