Fix C++ Boost incompatibility on Windows/MSVC

[yeswalrus]

Removes the #definition of __BYTE_ORDER on Windows which is unused by the rest of the Protobuf library (at least according to grep)

__LITTLE_ENDIAN and __BIG_ENDIAN are both undefined, meaning that
__BYTE_ORDER == __LITTLE_ENDIAN
and
__BYTE_ORDER == __BIG_ENDIAN both wind up evaluating to true.

This breaks boost/predef/other/endian.h, which checks for the existence of sys/param.h by looking for __BYTE_ORDER, and assumes that if __BYTE_ORDER is defined, then both __LITTLE_ENDIAN and __BIG_ENDIAN will be.

If these variables are required, a more complete windows shim of sys/param.h should be created.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok to test.

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[yeswalrus]

I signed it!

[googlebot]

CLAs look good, thanks!

[xfxyjwf]

Update the comment as well?

[yeswalrus]

Good catch, thanks.

[yeswalrus]

Ready to go.

[xfxyjwf]

Thanks! Could you merge the commits into one commit? I'll merge this pull request after that.

[yeswalrus]

Done!

[yeswalrus]

@xfxyjwf Ping.