runtime: avoid zeroing scavenged memory
On Linux, memory returned to the kernel via MADV_DONTNEED is guaranteed
to be zero-filled on its next use.
This commit leverages this kernel behavior to avoid a redundant software
zeroing pass in the runtime, improving performance.
Change-Id: Ia14343b447a2cec7af87644fe8050e23e983c787
GitHub-Last-Rev:
6c8df322836e70922c69ca3c5aac36e4b8a0839a
GitHub-Pull-Request: golang/go#76063
Reviewed-on: https://go-review.googlesource.com/c/go/+/715160
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>