[gdb] Fix style.indent in tclint.toml

The style.indent setting in gdb/tclint.toml is set to 4 spaces, which is an
approximation.

Starting with v0.6.2, tclfmt supports setting style.indent to mixed,<s>,<t>,
which allows us to specify the actual indentation scheme.

Set style.indent to mixed,4,8.

PR testsuite/33724
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33724