feat update byte_order.h for emscripten

# This file is part of Eigen, a lightweight C++ template library

# for linear algebra.

#

# Copyright (C) 2020 Arm Ltd. and Contributors

#

# This Source Code Form is subject to the terms of the Mozilla

# Public License v. 2.0. If a copy of the MPL was not distributed

# with this file, You can obtain one at http://mozilla.org/MPL/2.0/.

stages:

  - buildsmoketests

  - smoketests

  - build

  - test

variables:

  BUILDDIR: builddir

  EIGEN_CI_CMAKE_GENEATOR: "Ninja"

include:

  - "/ci/smoketests.gitlab-ci.yml"

  - "/ci/build.gitlab-ci.yml"

  - "/ci/test.gitlab-ci.yml"

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### Summary

<!-- Summarize the bug encountered concisely. -->

### Environment

<!-- Please provide your development environment here -->

- **Operating System** : Windows/Linux

- **Architecture** : x64/Arm64/PowerPC ...

- **Eigen Version** : 3.3.9

- **Compiler Version** : Gcc7.0

- **Compile Flags** : -O3 -march=native

- **Vector Extension** : SSE/AVX/NEON ...

### Minimal Example

<!-- If possible, please create a minimal example here that exhibits the problematic behavior.

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```cpp

//show your code here

```

### Steps to reproduce

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1. first step

2. second step

3. ... 

### What is the current *bug* behavior?

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### What is the expected *correct* behavior?

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### Relevant logs

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### Benchmark scripts and results

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### Anything else that might help

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- [ ] Have a plan to fix this issue.

### Describe the feature you would like to be implemented.

### Would such a feature be useful for other users? Why?

### Any hints on how to implement the requested feature?

### Additional resources

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### Reference issue

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### What does this implement/fix?

<!--Please explain your changes.-->

### Additional information

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[patterns]

*.sh = LF

*.MINPACK = CRLF

scripts/*.in = LF

debug/msvc/*.dat = CRLF

debug/msvc/*.natvis = CRLF

unsupported/test/mpreal/*.* = CRLF

** = native

[repository]

native = LF