upstream Aro and use it for translate-c instead of clang

[andrewrk]

Aro is a C compiler written in Zig by @Vexu and @ehaas.

This issue is for transitioning Zig's translate-c feature from being based on Clang to being based on Aro.

The transition will probably need to be done a little at a time, so probably a flag will be introduced to support both backends for a period of time. Another option would be that -Denable-llvm makes it always use Clang and not passing this option makes it always use Aro.