runtime: reduce syscall when call runtime.clone
runtime: reduce syscall when call runtime.clone

Change-Id: I3ea398fd86aae4c86557dd6fff65d90a6f756890
GitHub-Last-Rev: 4c295388f7b5e6768ffd2530337f78b4c75a9310
GitHub-Pull-Request: golang/go#40392
Reviewed-on: https://go-review.googlesource.com/c/go/+/244626
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,CAFxX/go,golang/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go