Don't default to -fno-exceptions -fno-rtti

[swolchok]

We need exceptions/RTTI for non-core-only use cases (see #7736).
Sending first version of this without patching any specific targets to restore these flags because I want to see if anything actually breaks.

[swolchok]

Stack from ghstack (oldest at bottom):

• -> Don't default to -fno-exceptions -fno-rtti #7746
• Don't repeat default for CMAKE_CXX_FLAGS_DEBUG #7743
• Don't clobber existing CMAKE_CXX_FLAGS_RELEASE #7742

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/7746

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❌ 2 New Failures

As of commit d47c434 with merge base cd0e584 ():

NEW FAILURES - The following jobs have failed:

• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t bf2c7456a0c025dfaeeab4ddf32d50b2b8b24e266bb6a767b5dbd44270fe9a76 /exec failed with exit code 2
• trunk / test-models-macos (cmake, emformer_transcribe, xnnpack-quantization-delegation, macos-m1-stable, 90) / macos-job (gh)
  RuntimeError: Missing out variants: {'quantized_decomposed::dequantize_per_tensor', 'quantized_decomposed::quantize_per_tensor'}

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[swolchok]

phi-3-mini and emformer-transcribe failures are from trunk, so it looks likely that this doesn't break anything.