Various armv7 related fixes (#5394)
* - Link with libatomic if needed
- Install pip differently so it doesn't clash with the system pip which may involve a wrapper script
- Remove ability to specify offset when Tensor allocates the data. The data prior to offset isn't accessible by anything.
- Fix use of offset in TensorOpTest to work on armv7 where it must be aligned to the type it points to.
- Fix ActivationOpNoInfTest.Softsign to allow for armv7 behavior
- Fix ReductionOpTest.ReduceMean_*keepdims to allow for armv7 floating point inaccuracy
* Address PR comments
