Enable CoreCLR contracts for Linux x64 by removing unconditional UNIX contract disablement

[Copilot]

CoreCLR contracts were globally disabled on UNIX, which kept Linux x64 excluded even after ambiguous contract syntax was removed in #126625. This change narrows that policy so Linux x64 CoreCLR builds can run with contracts enabled while preserving existing behavior elsewhere.

• Build configuration change

  • Updated /home/runner/work/runtime/runtime/eng/native/configurecompiler.cmake to stop defining DISABLE_CONTRACTS for CoreCLR + Linux + AMD64.
  • Kept DISABLE_CONTRACTS in place for all other UNIX targets.

• Scope control

  • Change is limited to compile-definition gating; no contract macro refactors and no unrelated runtime behavior changes.

• Resulting configuration shape

if(CLR_CMAKE_TARGET_UNIX)
  if(NOT (CLR_CMAKE_RUNTIME_CORECLR AND CLR_CMAKE_TARGET_LINUX AND CLR_CMAKE_TARGET_ARCH_AMD64))
    add_definitions(-DDISABLE_CONTRACTS)
  endif()
endif()

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