fix(arch): guard stdbool.h shims against C23 keyword conflict

[thecapn32]

GCC 15 defaults to -std=gnu23, where bool/true/false are language
keywords. The arm32 and riscv stdbool.h shims unconditionally
redefined them via enum/typedef, which fails to compile under C23.

• Only define bool/true/false when the compiler doesn't already
  provide them as C23 keywords, and skip entirely under C++
• Use #define bool _Bool (the real builtin type) instead of a
  typedef'd int8_t lookalike, matching glibc/newlib's stdbool.h
• Add the standard __bool_true_false_are_defined guard macro