[mac/arm] skip MappedMemoryTest that try to map w+x

macOS/arm is w^x, so these tests don't work. Fixes these failures:

  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.AllocAndRelease/5
  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.AllocAndReleaseHuge/5
  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.BasicWrite/5
  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.DuplicateNear/5
  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.EnabledWrite/3
  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.EnabledWrite/4
  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.EnabledWrite/5
  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.MultipleAllocAndRelease/5
  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.MultipleWrite/5
  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.SuccessiveNear/5
  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.UnalignedNear/5
  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.ZeroNear/5
  LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.ZeroSizeNear/5

Part of PR46647.