recentish Alpine don't love >64 vCPUs

I noticed that a VM on a racklette seemed to get stuck at booting 'Linux virt' when I gave it a bunch (140) vCPUs. if you look at per-processor load, a bunch of vCPUs end up sitting at 100% in guest code, but it never really makes progress towards booting.

I've narrowed this down to a Linux change somewhere between 6.12.31 (Alpine 3.22.0) and 6.12.38 (Alpine 3.22.1) but the changelogs for 6.12.33 to 6.12.36 are ... voluminous.

this is very easily reproducible on any helios system and propolis-standalone, you really do just need to set cpus = 66 or whatever. I was using 80 as a nice round number. I'm probably holding Linux wrong but I couldn't quickly get log lines out to the serial port while it's booting, so I'm not sure where it gets stuck. bit I did notice that all vCPUs get busy for a short period early in boot that seems to line up with smpboot: x86: Booting SMP configuration:, a line about all the CPUs, and then smp: Brought up 1 node, 64 CPUs.

I basically expect that this is https://github.com/oxidecomputer/propolis/issues/940 in some weird way, but it'd be good to be sure what this is or isn't.

Reproduce the issue in propolis-standalone with an Alpine virt guest and cpus = 66 or 80, comparing Alpine 3.22.0 (Linux 6.12.31) with 3.22.1 (Linux 6.12.38). Start by checking serial boot output around x86 SMP bring-up and the reported 64-CPU limit, then determine whether this is related to issue 940; done means the regression is explained and the affected behavior is fixed or clearly ruled out.