This is a minor refactoring to extract functions that resolve target_os
and target_cpu gn settings.

This is cleanup prior to adding support for host builds on Apple Silicon
Macs.

Issue: flutter/flutter#96745

This is a reland of #33224
That patch was reverted in #33231

On non-host Windows builds (e.g. android builds of gen_snapshot)
target_cpu is rewritten to either x86 or x64. However, dart_target_arch
should remain the original target_cpu (e.g. arm or arm64). The previous
version of this refactor inlined that logic into get_target_cpu() which
happens before we set dart_target_arch, thus incorrectly causing
gen_snapshot to be built to target x86 or x64 instead of the original
target architecture.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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