Skip tests on arm platforms.
The godoc tests require large amounts of memory, in excess of 700mb in -index mode which none of the arm builders have spare.
Because of their requirements the tests can be killed by the test runner leaving stray godoc processes spinning in swap trying to -index.
Change-Id: I1544d56e9d9aabbbaac21adeebfb9e2690bd2da5
Reviewed-on: https://go-review.googlesource.com/7540
Reviewed-by: Andrew Gerrand <adg@golang.org>