editorconfig: add settings for .s/.S files

We are starting to add assembler foe tests/tcg so lets make sure we
get the mode right.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

.editorconfig

@@ -26,6 +26,11 @@ file_type_emacs = makefile
 indent_style = space
 indent_size = 4
 
+[*.{s,S}]
+indent_style = tab
+indent_size = 8
+file_type_emacs = asm
+
 [*.{vert,frag}]
 file_type_emacs = glsl