plugin: skip building tests on linux/arm64

It appears that linux/arm64

https://build.golang.org/log/6808dbded6aebadf68cb65a0e30e4d1a62cd687b

fails with

/workdir/go/pkg/tool/linux_arm64/link: running gcc failed: exit status 1
/usr/bin/ld.gold: internal error in global, at ../../gold/aarch64.cc:4973
collect2: error: ld returned 1 exit status
FAIL plugin [build failed]

error. So stop building these tests on linux/arm64.

Fixes linux/arm64 build

Change-Id: I41eb3d9659f7967d80136513899a5203bbf03fb1
Reviewed-on: https://go-review.googlesource.com/c/151478


Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>