gc-stacks: Opt task stacks out of transparent huge pages (#64407)
When the kernel puts transparent huge pages into mappings that didn't ask
for them (THP `enabled=always`, the default on e.g. Arch and RHEL), the
first touch of an 8 MiB task stack faults in and zeroes a whole huge page.
Every started task then costs ~2 MiB of RSS, and spawn-heavy code spends
most of its time in page faults. Mark task stacks `MADV_NOHUGEPAGE`.
Assisted-by: Claude Code (Opus 5.5)
