arrow-cpp: Fix building x86_64-darwin on aarch_64-darwin (#193207)
When building x86_64-darwin emulated via Rosetta on aarch64-darwin, all
tests would fail with
> Illegal instruction: 4
this resolves that by never using Jemalloc on Darwin, since even though
`isAarch64` is false, it might “really” be aarch64-darwin.
authored by Andrew Marshall and committed by GitHub cc89e76d 859516a1