[gdb] Fix whitespace in *.[ly]

Add indent-with-non-tab for *.[ly] in gdb/.gitattributes.

Fix whitespace in the *.[ly] files in gdb, and add these files to the clean
list in gdb/contrib/check-whitespace-pre-commit.py.

Tested on x86_64-linux.

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