runtime: write barrier incorrect on wasm

[aclements]

The implementation of the write barrier on wasm incorrectly implements the "deletion" part of the barrier, and has since the code was introduced in Go 1.11. I have a fix. This issue is to track backports.

[gopherbot]

Change https://golang.org/cl/167743 mentions this issue: runtime: fix write barrier on wasm

[aclements]

@gopherbot, please backport to Go 1.12 and Go 1.11.

[gopherbot]

Backport issue(s) opened: #30872 (for 1.11), #30873 (for 1.12).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.

[aclements]

Thanks @gopherbot!