runtime: disable windowed Smhasher test on 32-bit systems
    
    This test tends to be flaky on 32-bit systems.
    There's not enough bits in the hash output, so we
    expect a nontrivial number of collisions, and it is
    often quite a bit higher than expected.
    
    Fixes #43130
    
    Change-Id: If35413b7c45eed778a08b834dacf98009ceca840
    Reviewed-on: https://go-review.googlesource.com/c/go/+/402456
    Run-TryBot: Keith Randall <khr@golang.org>
    TryBot-Result: Gopher Robot <gobot@golang.org>
    Reviewed-by: Ian Lance Taylor <iant@google.com>
    Reviewed-by: Keith Randall <khr@google.com>