arrow-cpp: Fix building x86_64-darwin on aarch_64-darwin (#193207)
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.
NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs