[gdb] Handle *.ac and *.m4 files in .gitattributes

[gdb] Handle *.ac and *.m4 files in .gitattributes

Since commit 52ca3d3fe61 ("toplevel: unify the GCC and GDB/binutils .editorconfig
files"), .editorconfig has settings for .ac and .m4 files:
...
[*.{ac,m4}]
indent_style = tab
indent_size = 2
trim_trailing_whitespace = true
...

There are no setting for those files in .gitattributes, so the whitespace
attribute defaults to trailing-space (shorthand for blank-at-eol,
blank-at-eof) and space-before-tab.

Since according to .editorconfig the indentation style is tab, add
indent-with-non-tab as well.

Since aclocal.m4 is generated, unset the whitespace attribute.  Likewise for
configure.

Probably, this eventually needs to be added to root level .gitattributes, but
for now try this in gdb*/.gitattributes.

Approved-By: Simon Marchi <simon.marchi@efficios.com>