Adds a missing restore of the app's stolen register value into the signal frame's ucontext when a signal interrupts an auto-restart syscall. In that case, we do not go through the full translation path and instead perform a custom translation, which is why we missed this step.
Adds asserts on several signal paths to help detect similar errors or regressions in the future.
Adds comments making it more clear what is in the stolen register's mcontext slot while in the cache.
Tested on the linux.eintr test which fails with the new asserts but without the fix. Additionally tested on the proprietary app where it was first encountered.
Fixes #4460 (closed)