Currently Flutter compiles gen_snapshot_x64 on x64 Intel LUCI bots. Apple will likely phase x64 machines out of their product line in the next few years, which will make it increasingly difficult to keep enough of these machines up to maintain builder capacity (due to inability to source new machines, security updates becoming less frequent, etc). Enable the capability to compile x64 binaries on arm64 Macs.

cc @mraleph