Switch iOS gen_snapshot from multi-arch binary to multiple binaries

[liamappelbe]

Description

As of flutter/engine#10430, we no longer combine the iOS armv7 and arm64 gen_snapshots into a single binary. Instead they're uploaded as two separate binaries in the same directory (https://chromium-review.googlesource.com/c/chromium/tools/build/+/1731443). So this change chooses the gen_snapshot to match the iosArch.

Related Issues

#22598

Tests

I built the flutter examples in this mode using Xcode 10 and verified that they work on an iPhone 4S.

Manual roll

git log b41c172..eac7ef0 --no-merges --oneline
flutter/engine@eac7ef065 Add copy_gen_snapshots.py tool (flutter/engine#10430)
flutter/engine@c3cc104e1 Fix mac gen_snapshot uploader (flutter/engine#10423)
flutter/engine@310bc030b Make kernel compiler use host toolchain (flutter/engine#10419)
flutter/engine@9fca3c744 Use simarm_x64 when targeting arm (flutter/engine#10010)
flutter/engine@bf9288597 [fuchsia] Add kernel_compiler target in build_fuchsia script (flutter/engine#10403)

[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. While there are exceptions to this rule, if this patch modifies code it is probably not an exception.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

/cc @dnfield

[codecov]

Codecov Report

Merging #37445 into master will increase coverage by 1.62%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master   #37445      +/-   ##
==========================================
+ Coverage   54.07%   55.69%   +1.62%     
==========================================
  Files         193      193              
  Lines       18018    18017       -1     
==========================================
+ Hits         9743    10035     +292     
+ Misses       8275     7982     -293

Flag	Coverage Δ
#flutter_tool	55.69% <0%> (+1.62%)	⬆️

Impacted Files	Coverage Δ
packages/flutter_tools/lib/src/base/build.dart	73.71% <0%> (+27.12%)	⬆️
...lutter_tools/lib/src/android/android_emulator.dart	32.65% <0%> (-30.62%)	⬇️
packages/flutter_tools/lib/src/cache.dart	41.99% <0%> (-0.73%)	⬇️
packages/flutter_tools/lib/src/vmservice.dart	37.15% <0%> (-0.31%)	⬇️
packages/flutter_tools/lib/src/base/logger.dart	82.44% <0%> (+0.38%)	⬆️
packages/flutter_tools/lib/src/base/process.dart	80.46% <0%> (+0.78%)	⬆️
packages/flutter_tools/lib/src/base/version.dart	83.33% <0%> (+2.38%)	⬆️
packages/flutter_tools/lib/src/device.dart	57.22% <0%> (+4.21%)	⬆️
...ackages/flutter_tools/lib/src/commands/config.dart	85.89% <0%> (+12.82%)	⬆️
packages/flutter_tools/lib/src/base/utils.dart	92.57% <0%> (+13.36%)	⬆️
... and 5 more

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[cbracken]

We should add a GenSnapshot test that checks that we issue the run command with the appropriate binary name.