Previously, the `gen_snapshot_arm64` and `gen_snapshot_x64` binaries
used by the tool were all built for x64 architecture. As such,
developers building apps with Flutter rely on Rosetta translation with
every build.

This refactors the gen_snapshot build rules on macOS hosts to
consistently produce `gen_snapshot_arm64` and `gen_snapshot_x64`
binaries with the target architecture of the build but with both host
architectures.

Differences introduced in arm64 host build:

Prior to this patch we emitted:
* gen_snapshot_arm64 (arch: x64, target_arch: simarm64)

After this patch, we emit:
* gen_snapshot_arm64 (arch: x64, target_arch: simarm64)
* gen_snapshot_arm64 (arch: arm64, target_arch: arm64)

Differences introduced in x64 host build:

Prior to this patch we emitted:
* gen_snapshot_x64 (arch: x64, target_arch: x64)

After this patch, we emit:
* gen_snapshot_x64 (arch: x64, target_arch: x64)
* gen_snapshot_x64 (arch: arm64, target_arch: simx64)

Note that host builds on macOS currently default to a host architecture
of x64 (can be overridden via `--force-mac-arm64`) regardless of host
architecture and thus, the build itself relies on Rosetta translation
when invoked on Apple Silicon arm64 hardware. This is to ensure a
consistent build in CI regardless of bot architecture.
See: https://github.com/flutter/engine/blob/6fa734d686888a39add026a2a98d6ec311c23efb/tools/gn#L502-L505

Issue: flutter/flutter#101138
Issue: flutter/flutter#69157

Related issue: flutter/flutter#103386