[vm, simarm64] Extract CalleeRegisterVerifier class

[eseidel]

Extract the callee-saved register save/verify/restore logic from Simulator::Call into a RAII CalleeRegisterVerifier class.

• On construction, saves callee-saved register values and overwrites them with a known value
• On destruction, asserts the known value is preserved and restores originals
• Reduces ~25 lines of inline code in Simulator::Call to 3 lines
• No behavioral change

[eseidel]

This is a small clean up that we did in our fork of Dart that I figured I should upstream.

[copybara-service]

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[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/491682 has been updated with the latest commits from this pull request.