Arm backend: Use padding for the base64 coded ETDump string

[zingo]

This fix "base64: invalid input" errors you got sometimes when converting it to a file.

This change also avoid using ' ', '\r', and '\n' in the base64 string for a cleaner string.

cc @digantdesai @freddan80 @per @oscarandersson8218

[pytorch-bot]

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✅ You can merge normally! (3 Unrelated Failures)

As of commit 4c63aa8 with merge base 72580d2 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• trunk / unittest-release / macos / macos-job (gh) (similar failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Apple / build-demo-ios / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 65
• pull / unittest / macos / macos-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq

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