libstdc++: Add .editorconfig files

These config files set default formatting behaviour for a large number
of common editors, see https://editorconfig.org

The root=true setting in libstdc++-v3/.editorconfig prevents looking in
parent directories for additional settings. If we add a .editorconfig at
the top-level we might want to use root=true there instead, and allow
libstdc++-v3/.editorconfig to inherit some some settings from there (and
only override things we want to do differently).

libstdc++-v3/ChangeLog:

	* .editorconfig: New file.
	* include/std/.editorconfig: New file.

libstdc++-v3/.editorconfig

@@ -0,0 +1,38 @@
+# top-most EditorConfig file for libstdc++-v3
+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
+
+[*.xml]
+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

libstdc++-v3/include/std/.editorconfig

@@ -0,0 +1,10 @@
+# Inherit settings from EditorConfig files above this dir.
+root = false
+
+# All files under this directory are C++ headers.
+[*]
+charset = utf-8
+indent_style = tab
+indent_size = 2
+tab_width = 8
+trim_trailing_whitespace = true