An arch should select this symbol if it can support kernel stacks
in vmalloc space. This means:

- vmalloc space must be large enough to hold many kernel stacks.
This may rule out many 32-bit architectures.

- Stacks in vmalloc space need to work reliably. For example, if
vmap page tables are created on demand, either this mechanism
needs to work while the stack points to a virtual address with
unpopulated page tables or arch code (switch_to() and switch_mm(),
most likely) needs to ensure that the stack's page table entries
are populated before running on a possibly unpopulated stack.

- If the stack overflows into a guard page, something reasonable
should happen. The definition of "reasonable" is flexible, but
instantly rebooting without logging anything would be unfriendly.