osdep/compiler: add MP_CKD_MUL

Add a new MP_CKD_MUL macro which does a multiplication with overflow
checking, backed by either the C23 ckd_mul standard function, or the
GCC/Clang __builtin_mul_overflow otherwise.

osdep/compiler.h

@@ -7,6 +7,7 @@
 #define MP_EXPAND_ARGS(...) __VA_ARGS__
 
 #if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#include <stdckdint.h>
 #define MP_NORETURN [[noreturn]]
 #define MP_FALLTHROUGH [[fallthrough]]
 #define MP_WARN_UNUSED_RESULT [[nodiscard]]
@@ -84,4 +85,10 @@
 #define MP_SCANF_ATTRIBUTE(a1, a2)
 #endif
 
+#if defined(__STDC_VERSION_STDCKDINT_H__) && __STDC_VERSION_STDCKDINT_H__ >= 202311L
+#define MP_CKD_MUL(result, a, b) ckd_mul(result, a, b)
+#elif __has_builtin(__builtin_mul_overflow)
+#define MP_CKD_MUL(result, a, b) __builtin_mul_overflow(a, b, result)
+#endif
+
 #endif