Intel's (Cordova-compatible) Crosswalk project supports SIMD:

https://crosswalk-project.org/

And the link to the sources is the first link in the first reply to the original post:

https://github.com/crosswalk-project/v8-crosswalk/commits/ma...

The post seems to mention crossplatform support for SIMD.js. Does Intel's version work on ARM, too?

Yes - SIMD.js is (for now) seemingly only going to support SIMD features which can be implemented on two platforms. For example, 256 and 512-bit vectors have been proposed for SIMD.js, but the word is they won't be put into the spec until they can be implemented without a polyfill on two platforms (x86 and ARM).