binutils-gdb/git: Handle *.ac and *.m4 files in .gitattributes

binutils-gdb/git: 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.