Reverts "[macOS] Generate universal gen_snapshots (#52885)"

[auto-submit]

Reverts: #52885

Initiated by: cbracken

Reason for reverting: while this patch worked fine, it pushed the mac build bot over its time limit. Previous builds were just squeaking under the wire but seeing timeouts on the mac_host_engine host_release shard after this commit. e.g. https://ci.chromium.org/ui/p/flutter/builders/prod/Mac%20mac_host_engine/10667/overview

Looking at ci.yaml I see the timeout is set to 240, but the timeout on the failing shard is c

Original PR Author: cbracken

Reviewed By: {jmagman}

This change reverts the following previous change:
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 as universal binaries with both host architectures.

arm64 host build

Prior to this patch we emitted:

• gen_snapshot_arm64 (arch: x64, target_arch: simarm64)

After this patch, we emit:

• artifacts_x64/gen_snapshot_arm64 (arch: x64, target_arch: simarm64)
• artifacts_arm64/gen_snapshot_arm64 (arch: arm64, target_arch: arm64)
• gen_snapshot_arm64 (universal binary composed of both of the above)

x64 host build

Prior to this patch we emitted:

• gen_snapshot_x64 (arch: x64, target_arch: x64)

After this patch, we emit:

• artifacts_x64/gen_snapshot_x64 (arch: x64, target_arch: x64)
• artifacts_arm64/gen_snapshot_x64 (arch: arm64, target_arch: simx64)
• gen_snapshot_x64 (universal binary composed of both of the above)

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:

engine/tools/gn

Lines 502 to 505 in 6fa734d

	# TODO(cbracken):
	# https://github.com/flutter/flutter/issues/103386
	if get_host_os() == 'mac' and not args.force_mac_arm64:
	gn_args['host_cpu'] = 'x64'

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

Related issue: flutter/flutter#103386

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