sys/_types.h: recognise char8_t as a builtin type in C++20

[fuz]

Summary

Unlike in C23 where it's a typedef, char8_t is a built in type in
C++20. Recognise it as such.

PR: 291449
Reported by: Tomoaki AOKI <[e-mail]>

Test Plan

system builds fine with this change, www/qt6-webengine builds too.

[markj]

Approved