as31: Add .gitignore file to keep autogenerated files out of git

as31/.gitignore

@@ -0,0 +1,11 @@
+Makefile
+as31/Makefile
+as31/as31
+as31/as31.o
+as31/emitter.o
+as31/lexer.o
+as31/parser.o
+as31/run.o
+as31/symbol.o
+config.log
+config.status