OpenBSD is the only platform that uses this support by default,
and it did not work because while OpenBSD supports alloca, it does
not include alloca.h.
This should be backported to Ruby 2.7.
From George Koehler
We use COROUTINE_LIMITED_ADDRESS_SPACE to select platforms where address
space is 32-bits or less. Fiber pool implementation enables more book
keeping, and reduces upper limits, in order to minimise address space
utilisation.
Nobuyoshi Nakada