crypto/internal/bigmod: flag amd64 assembly as noescape [1.20 backport]

[gopherbot]

@FiloSottile requested issue #58501 to be considered for backport to the next 1.20 minor release.

@gopherbot please open a Go 1.20 backport. This is arguably partially mitigating a performance regression in Go 1.20, it's a "bug" in new code introduced in Go 1.20, and the patch is as safe as it gets.

[gopherbot]

Change https://go.dev/cl/471855 mentions this issue: [release-branch.go1.20] crypto/internal/bigmod: flag amd64 assembly as noescape

[gopherbot]

Closed by merging ef79380 to release-branch.go1.20.