os/exec: revert macOS __fork fix before point release [1.18 backport]

[gopherbot]

@rsc requested issue #57688 to be considered for backport to the next 1.18 minor release.

@gopherbot please backport WTF

[gopherbot]

Change https://go.dev/cl/461116 mentions this issue: [release-branch.go1.18] runtime: revert "call __fork instead of fork on darwin"

[gopherbot]

Closed by merging 87105e5 to release-branch.go1.18.