Previously we would prevent any operating system not providing the MEM_NORESERVE flag
from using the two-step allocator. Afterall, Linux will reserve swap-space for
a mapping unless this flag is given, which is most certainly not what we want.
However, it seems that FreeBSD provides the reservation-only mapping behavior
that we expect despite not providing the MEM_NORESERVE macro. In fact, it
provided the macro until 2014, when it was removed on account of not being
implemented in the kernel. However, empirical evidence suggests that just plain
mmap does what we want.