Repository: flutter
Issue: `gen_snapshot` failure on M1 Mac host builds during CPU feature detection.
Labels: engine,platform-mac,P2,team-engine,triaged-engine

This error occurs when attempt to build and run engine artifacts natively on an M1 Mac without emulation.

```
[3/102] python3 ../../build/gn_run_binary.py clang_arm64/gen_snapshot --snapshot_kind=core --enable_mirrors=false --vm_snapshot_data=/Users/buzzy/VersionControlled/flutter_engine/src/out/host_debug_unopt/gen/flutter/lib/snapshot/vm_isolate_snapshot.bin --vm_snapshot_instructions=/Users/buzzy/VersionControlled/flutter_engine/src/out/host_debug_unopt/gen/flutter/lib/snapshot/vm_snapshot_instructions.bin --isolate_snapshot_data=/Users/buzzy/VersionControlled/flutter_engine/src/out/host_debug_unopt/gen/flutter/lib/snapshot/isolate_snapshot.bin --isolate_snapshot_instructions=/Users/buzzy/VersionControlled/flutter_engine/src/out/host_debug_unopt/gen/flutter/lib/snapshot/isolate_snapshot_instructions.bin --enable_asserts /Users/buzzy/VersionControlled/flutter_engine/src/out/host_debug_unopt/flutter_patched_sdk/platform_strong.dill
FAILED: gen/flutter/lib/snapshot/vm_isolate_snapshot.bin gen/flutter/lib/snapshot/vm_snapshot_instructions.bin gen/flutter/lib/snapshot/isolate_snapshot.bin gen/flutter/lib/snapshot/isolate_snapshot_instructions.bin 
python3 ../../build/gn_run_binary.py clang_arm64/gen_snapshot --snapshot_kind=core --enable_mirrors=false --vm_snapshot_data=/Users/buzzy/VersionControlled/flutter_engine/src/out/host_debug_unopt/gen/flutter/lib/snapshot/vm_isolate_snapshot.bin --vm_snapshot_instructions=/Users/buzzy/VersionControlled/flutter_engine/src/out/host_debug_unopt/gen/flutter/lib/snapshot/vm_snapshot_instructions.bin --isolate_snapshot_data=/Users/buzzy/VersionControlled/flutter_engine/src/out/host_debug_unopt/gen/flutter/lib/snapshot/isolate_snapshot.bin --isolate_snapshot_instructions=/Users/buzzy/VersionControlled/flutter_engine/src/out/host_debug_unopt/gen/flutter/lib/snapshot/isolate_snapshot_instructions.bin --enable_asserts /Users/buzzy/VersionControlled/flutter_engine/src/out/host_debug_unopt/flutter_patched_sdk/platform_strong.dill
../../third_party/dart/runtime/vm/cpuinfo_macos.cc: 42: error: unreachable code
version=2.15.0-149.0.dev (dev) (Fri Sep 24 17:51:03 2021 -0700) on "macos_x64"
pid=8650, thread=259, isolate_group=(nil)(0x0), isolate=(nil)(0x0)
isolate_instructions=0, vm_instructions=0
  pc 0x00000001045cb128 fp 0x000000016bafab10 dart::Profiler::DumpStackTrace(void*)+0x90
  pc 0x0000000104335ac0 fp 0x000000016bafab30 dart::Assert::Fail(char const*, ...) const+0x28
  pc 0x000000010448d43c fp 0x000000016bafaf70 dart::CpuInfo::FieldContains(dart::CpuInfoIndices, char const*)+0xb0
  pc 0x000000010448d180 fp 0x000000016bafaf90 dart::HostCPUFeatures::Init()+0x64
  pc 0x000000010448d858 fp 0x000000016bafb1b0 dart::Dart::DartInit(unsigned char const*, unsigned char const*, _Dart_Isolate* (*)(char const*, char const*, char const*, char const*, Dart_IsolateFlags*, void*, char**), bool (*)(void**, char**), void (*)(void*, void*), void (*)(void*, void*), void (*)(void*), void (*)(), void* (*)(char const*, bool), void (*)(unsigned char**, long*, void*), void (*)(void const*, long, void*), void (*)(void*), bool (*)(unsigned char*, long), _Dart_Handle* (*)(), bool, Dart_CodeObserver*)+0x2e0
  pc 0x000000010448ddf8 fp 0x000000016bafb230 dart::Dart::Init(unsigned char const*, unsigned char const*, _Dart_Isolate* (*)(char const*, char const*, char const*, char const*, Dart_IsolateFlags*, void*, char**), bool (*)(void**, char**), void (*)(void*, void*), void (*)(void*, void*), void (*)(void*), void (*)(), void* (*)(char const*, bool), void (*)(unsigned char**, long*, void*), void (*)(void const*, long, void*), void (*)(void*), bool (*)(unsigned char*, long), _Dart_Handle* (*)(), bool, Dart_CodeObserver*)+0x74
  pc 0x0000000104cf8e8c fp 0x000000016bafb290 Dart_Initialize+0x68
  pc 0x000000010430874c fp 0x000000016bafb430 dart::bin::main(int, char**)+0x650
  pc 0x0000000189281430 fp 0x000000016bafb450 start+0x4
-- End of DumpStackTrace
```

This is building natively on an M1 Mac.