os/exec: subprocesses on ios failing frequently with SIGTERM since CL 459176

[bcmills]

#!watchflakes
post <- goos == "ios" && (`signal: segmentation fault` || `fork/exec .*: cannot allocate memory`)

The ios-arm64-corellium builder started failing frequently as of CL 459176, which attempted another fix for #56784 in light of #57263.

(CC @golang/ios, @rsc, @cherrymui)

[heschi]

Let's track this as a potential release blocker; this might be an iOS problem in which case perhaps we can skip __fork there, or maybe something's wrong with Darwin overall in which case we want to know that.

[gopherbot]

Change https://go.dev/cl/460476 mentions this issue: cmd/link, runtime: Apple libc atfork workaround take 3

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
post <- goos == "ios" && (`signal: segmentation fault` || `fork/exec .*: cannot allocate memory`)

2023-01-09 21:50 ios-arm64-corellium go@64519baf runtime/internal/atomic.test [build] (log)

/tmp/workdir-host-ios-arm64-corellium-ios/go/pkg/tool/ios_arm64/link: running /var/root/bin/clangwrap failed: signal: segmentation fault

2023-01-09 22:50 ios-arm64-corellium go@0202ad0b testing/quick.test [build] (log)

/tmp/workdir-host-ios-arm64-corellium-ios/go/pkg/tool/ios_arm64/link: running /var/root/bin/clangwrap failed: signal: segmentation fault

2023-01-10 17:59 ios-arm64-corellium go@82f09b75 compress/zlib.test [build] (log)

/tmp/workdir-host-ios-arm64-corellium-ios/go/pkg/tool/ios_arm64/link: running /var/root/bin/clangwrap failed: signal: segmentation fault

— watchflakes

[bcmills]

The above failures predate the fix CL (see #57632).