Linux gives the main thread a name based on the process name, and also
we need to tolerate an extra slash on the `<compiler-generated>` filename
for some reason.
rdar://121430255
Always put `registers` in the thread record, and always use the `threads`
top level key, even if we ask for only the crashed thread. Also add an
`omittedThreads` key.
rdar://121430255
