Purpose of macros SIMDUTF_CAN_ALWAYS_RUN_XYZ

[WojciechMula]

I see these macros are defined in simdutf/src/simdutf/*.h, but never used in the project. Should they be kept? TBH, don't know what was the initial purpose.

[lemire]

These macros could be used to remove some bloat and speed up the builds. For example, if SIMDUTF_CAN_ALWAYS_RUN_ARM64 is set, then you do not need to include the fallback kernel when building the library.

We don't use them right now so I think that they can be safely removed.

[WojciechMula]

Sound reasonable. IMHO better would be to utilize them. Although I would go for split into major architectures: x86/ARM/PowerPC and then do some global ifdef-magic.