sys/stdint.h: add C23 _WIDTH macros

[fuz]

Summary

The platform-dependent macros are added to the various
_stdint.h headers, those that are always the same are
added directly to _stdint.h.

We may want to move the definitions for WCHAR_* and
WINT_* out of the platform header files as those are
always the same.

Test Plan

see following DR

[markj]

sys/arm64/include/_stdint.h

167

Presumably this one should be INT64_WIDTH.

I'm not sure how to handle CHERI here, but @brooks can hopefully comment on that.