cmd/compile: go binaries not working on exynos 64 bit CPUs [1.11 backport]

[gopherbot]

@eliasnaur requested issue #28431 to be considered for backport to the next 1.11 minor release.

The easy sledgehammer is disabling the optimization if runtime.GOOS == "android".

@gopherbot please consider this for backport to 1.11. It's an unavoidable crash on a popular Android phone and easy to avoid from Go.

[gopherbot]

Change https://golang.org/cl/149557 mentions this issue: [release-branch.go1.11] runtime: avoid arm64 8.1 atomics on Android

[bradfitz]

Leaving for @randall77 to decide (whether to approve the cherry-pick).

[bradfitz]

(Marked approved per Keith's comment on the CL)

[gopherbot]

Closed by merging f3b9f36 to release-branch.go1.11.