www/pomerium-envoy-custom: Attempt to fix LuaJIT build on aarch64

On FreeBSD aarch64, Bazel resolves the CPU constraint to its canonical
name @platformscpu:aarch64 rather than the @platformscpu:arm64
alias. Add aarch64 to the HOST_CONSTRAINTS check in luajit.bzl so
the platform is correctly detected on arm64 FreeBSD.