Some possible progress here.

Thread on the previous error experienced under Swift 5.6: https://forums.swift.org/t/build-crash-when-building-in-qemu-using-new-swift-5-6-arm64-image/56090

The error has changed under Swift 5.9: https://forums.swift.org/t/swift-runtime-unable-to-suspend-thread-when-compiling-in-qemu/67676

Resolved in #351, error still occurs occasionally and has changed to:

swift-runtime: unable to suspend thread 915
swift-runtime: unable to suspend thread 1595
swift-runtime: unable to suspend thread 915
swift-runtime: unable to suspend thread 1623

*** Signal 5: Backtracing from 0x55052c0490... failed ***

Trace/breakpoint trap (core dumped)

But it is less frequent, and once we have a build in the cache then it doesn't need building again, so let's try and resolve this for now.