(Thanks partly to Node) V8 works on Power (LE & BE, Linux & AIX), I'm pretty sure it works on MIPS (https://github.com/v8/v8/tree/master/src/mips64), and you can even get in on z/OS (https://developer.ibm.com/node/sdk/ztp/)!
Not sure about Chromium support on those platforms, but V8 support is pretty impressive.
Except anything MIPS-based. Or Power. Or in fact anything that isn't x86 or ARM.
And it's not just a matter of compiling it for those platforms. There's a bunch of architecture-specific porting that would have to be done (e.g. you _have_ to implement a V8 backend; there is no platform-independent way to run V8 just with a C++ compiler).