include/stdckdint.h: make the header compatible with C++

[kib]

Summary

by removing the cast to _Bool.  The type is not defined for C++,
and the specification from gcc info states that the return
type of the  __builtin_{add,sub,mul}_overflow() is bool already.

This is done instead of including stdbool.h to avoid namespace
pollution, since defining bool from stdckdint.h simingly not
sanctioned by ISO/IEC 9899:2024.

PR:     290299