top-level: Add .editorconfig file

This config file sets default formatting behaviour for a large number
of common editors, see https://editorconfig.org

It also ensures that https://forge.sourceware.org formats GCC code
correctly, because it defaults to tab_width=4 but will respect a
.editorconfig file if present in the repo.

ChangeLog:

	* .editorconfig: New file.

.editorconfig

@@ -0,0 +1,38 @@
+# top-most EditorConfig file for gcc
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+
+[*.h,*.cc]
+charset = utf-8
+indent_style = tab
+indent_size = 2
+tab_width = 8
+trim_trailing_whitespace = true
+
+[Makefile*,ChangeLog*]
+indent_style = tab
+indent_size = 8
+trim_trailing_whitespace = true
+
+[ChangeLog*]
+charset = utf-8
+
+[*.texi]
+charset = utf-8
+indent_size = 2
+tab_width = 8
+trim_trailing_whitespace = true
+
+[*.py]
+indent_style = space
+indent_size = 4
+trim_trailing_whitespace = true
+
+[*.exp]
+indent_style = tab
+indent_size = 4
+tab_width = 8
+trim_trailing_whitespace = true