Support for SPV_KHR_float_controls

This would allow annotating functions to specify the new execution modes such as rounding modes. Not sure how this would be done syntactically to keep the impact on the language minimal. Maybe something like [C++ attributes](https://en.cppreference.com/w/cpp/language/attributes)?

The issue names no files, tests, or entry points. Start by reviewing the compiler's existing execution-mode and function-annotation handling, then compare it with SPV_KHR_float_controls and the proposed C++-attribute-like syntax. Done means an agreed minimal syntax and implementation plan for annotating functions with the requested execution modes.

Written by the indexing model from the issue text.