x/build: golang.org/x/arch/arm64/arm64asm tests failing

[s-mang]

for:

• darwin amd64 10.2
• linux-arm64-buildlet
• linux-arm64-packet

since @williamweixiao 's commit:
56b62c7

darwin failures are looking like:
2017/08/16 17:55:34 exit status 1
FAIL cmd/vendor/golang.org/x/arch/arm64/arm64asm 0.066s

linux-arm64 failures looking like:
--- FAIL: TestObjdumpARM64Testdata (2.06s)
ext_test.go:195: decode(c3425cf8) = "ldur x3, [x22,#-60]", 4, want "ldr x3, [x22,#-60]", 4

ext_test.go:195: decode(7c71db78) = "ldursh w28, [x11,#-73]", 4, want "ldrsh w28, [x11,#-73]", 4

...

cc @jessfraz @cybrcodr @zombiezen @aclements @ianlancetaylor
(no idea who to ping for this)

[cherrymui]

dup of issue #21486.
I sent CL https://go-review.googlesource.com/c/56450 to fix the darwin part.