Alternative syntax for PowerPC

In the assembly output for PowerPC by default only a bunch of numbers like `rlwinm 7, 3, 22, 27, 31` is seen. I was surprised by that unreadable syntax. It turns out [there's the `-mregnames` option in GCC and AT](https://stackoverflow.com/q/44909835/995714) for turning the register names on to differentiate with numbers

So I suggest doing something like the Intel/AT&T syntax toggling. We already have the facilities for that in x86, therefore just adding a few lines to the configuration file is enough, and then rename the button in PowerPC mode

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Unfortunately Clang doesn't support that flag. I'm going to file a feature request to them. But while waiting for that to happen I think we can just convert numbers to register names ourselves