Add .editorconfig to src/rustllvm

... which uses 2 space indent instead of common 4 spaces.

src/rustllvm/.editorconfig

@@ -0,0 +1,6 @@
+[*.{h,cpp}]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+indent_style = space
+indent_size = 2