Mysterious error and no time or easy machine access to investigate
now, so disabling the -buildmode=plugin test on arm64. (The arm
version is working as expected.)

Updates #17138

Change-Id: I4cc56ddf47e7597213462e48d4934a765168bd07
Reviewed-on: https://go-review.googlesource.com/29363Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>