Add target configuration to compiler for specifying assumed/required features

[benvanik]

Right now we are hardcoding which backends we target and assuming the same parameters for all of them. To allow benchmarking of changes that may require specific target capabilities (Vulkan/SPIR-V extensions, device limits, etc) we should be able to specify these as flags to the compiler and produce a variety of executables. At runtime we should then match against those to select the best suited for the given runtime configuration and allow overrides to make it easier to compare.

[benvanik]

Some of this work is happening in the Vulkan dialect with the target_env attributes, however we'll want something more general that wraps that so that we have a unified way of specifying targets across all backends.

[antiagainst]

+1. I think I'll be able to get to this when trying to come up with a real case for demo purpose. Assigning to myself for now.