[eigen] arm/neon inside eigen internals

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(mostly aimed at Konstantinos I guess)

In the recent patch providing Neon support (Thanks!), i have some questions regarding the use of __ARM_NEON__

1) Detection of architecture should be done in Core only, and in other files, we should only use things like EIGEN_VECTORIZE_NEON. This allows, for example, the user to define EIGEN_DONT_VECTORIZE and have indeed his code not making use of simd

2) For ABI reason, as explained in Eigen/src/Core/util/Macros.h:40, i think the line 42 should use __arm__ instead of __ARM_NEON__ or EIGEN_VECTORIZE_NEON

I first thought about patching/cleaning this myself, but i feel uncomfortable doing so because:

1) I want to know what Konstantinos thinks about it
2) I'm not sure ABI compatibility is such a hard requirement on embedded platforms  such as ARM
3) in some places (like Eigen/src/Core/util/Memory.h:426 or Eigen/src/Core/util/XprHelper.h:92) i'm not sure whether __ARM_NEON__ should be replaced by __arm__ or EIGEN_VECTORIZE_NEON. That is, should this codepath be used on all arm platforms (think 'abi') or only in the neon case.

Thomas Capricelli <orzel@xxxxxxxxxxxxxxx>

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