sys: copying .gitattributes to all subrepositories

Fixes #9281

Change-Id: Ia4bb2e9af9f660f42202bb1d07920578cd0f9c3e
Reviewed-on: https://go-review.googlesource.com/2076Reviewed-by: Minux Ma <minux@golang.org>

.gitattributes

+# Treat all files in this repo as binary, with no git magic updating
+# line endings. Windows users contributing to Go will need to use a
+# modern version of git and editors capable of LF line endings.
+#
+# We'll prevent accidental CRLF line endings from entering the repo
+# via the git-review gofmt checks.
+#
+# See golang.org/issue/9281
+
+* -text