Arm backend: Fix Direct Drive release build

[perheld]

This got mixed up when moving things into a preset cmake file and the -UNDEBUG that is needed to build Release never got set.

Signed-off-by: per.held@arm.com
Change-Id: I8022efa315c9f970a29b6d6bc4a9bee0be091f8b

cc @digantdesai @SS-JIA @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell

[pytorch-bot]

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❌ 4 New Failures

As of commit 1d69ab2 with merge base 1f4ad07 ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 9c5ef94335ebf98d01ec6d00ad12b245732daeb03c164415db4db4767b0a6276 /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t cedbd8a6a45f0cb64da14d6c848ce204bc80683819a1348c586d5a0cca849622 /exec failed with exit code 1
• pull / test-selective-build-linux / linux-job (gh)
  AttributeError: module 'urllib' has no attribute 'URLopener'
• pull / unittest-arm-backend-with-no-deps (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t d22f9becee01287bef4289532d310611c8fb54b6a3cbd2b133a18668ef0dcc09 /exec failed with exit code 1

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[Copilot]

Pull request overview

Fixes Arm Ethos-U Linux “Direct Drive” Release builds by ensuring -UNDEBUG is applied without clobbering existing Release compile flags after the preset refactor.

Changes:

• Append -UNDEBUG to existing CMAKE_C_FLAGS_RELEASE / CMAKE_CXX_FLAGS_RELEASE rather than replacing them.
• Force-set the cached Release flag variables so the preset reliably affects Release builds.

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