macOS: avoid calling pthread_self() twice

[kubo39]

No description provided.

[rust-highfive]

r? @shepmaster

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[rust-highfive]

The job x86_64-gnu-llvm-7 of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

2020-02-02T12:29:53.6992893Z ========================== Starting Command Output ===========================
2020-02-02T12:29:53.6994619Z [command]/bin/bash --noprofile --norc /home/vsts/work/_temp/63e80b83-af7f-403f-9f5c-9845f9d19677.sh
2020-02-02T12:29:53.6994658Z 
2020-02-02T12:29:53.6999317Z ##[section]Finishing: Disable git automatic line ending conversion
2020-02-02T12:29:53.7006047Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68767/merge to s
2020-02-02T12:29:53.7007654Z Task         : Get sources
2020-02-02T12:29:53.7007690Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-02-02T12:29:53.7007848Z Version      : 1.0.0
2020-02-02T12:29:53.7007883Z Author       : Microsoft
---
2020-02-02T12:29:54.7002577Z ##[command]git remote add origin https://github.com/rust-lang/rust
2020-02-02T12:29:54.7016133Z ##[command]git config gc.auto 0
2020-02-02T12:29:54.7018875Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2020-02-02T12:29:54.7021230Z ##[command]git config --get-all http.proxy
2020-02-02T12:29:54.7031788Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/68767/merge:refs/remotes/pull/68767/merge

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[JohnCSimon]

Ping from triage: @kubo39 - the build has failed.
Your code is for MacOS only but it's failing under Linux x86_64-gnu-llvm-7 ....

[shepmaster]

This seems reasonable, but can you share a little about why this change is important or why you noticed it?

[shepmaster]

You can go ahead and squash your two commits together now as well.

[kubo39]

I noticed this while reading the code. To be honest, this is not so important change(maybe slightly faster).

[shepmaster]

@bors r+ rollup

[bors]

📌 Commit 67068f3 has been approved by shepmaster

[shepmaster]

Thanks!