opcodes: add a .gitattributes file for aarch64 autogenerated file exceptions

The autogenerated files in opcodes use spaces for indentation.
Changing that would be a lot of work to little benefit, so add a local
override to the white-space rules, so patches apply cleanly.

opcodes/.gitattributes

@@ -0,0 +1,3 @@
+# The aarch64 generated files use spaces for indentation, so we need to
+# overrule the top-level whitespace rule.
+aarch64*-2.*	whitespace=space-before-tab,trailing-space