[Feat] Configure global state at compile-time

[mohamed]

This patch adds preprocessor facilities to enable choosing between global state (SoftFloatv3 default) and adding a "state" parameter to all functions. This has been discussed already by John Hauser at: https://github.com/ucb-bar/berkeley-softfloat-3/issues/5

This patch takes the position of enabling both options using a preprocessor macro named "SOFTFLOAT_USE_GLOBAL_STATE". If the macro is defined, then it uses global state as it does already. If the macro is undefined, then it adds a state parameter which is a struct defined in softfloat_types.h