[nrf fromlist] lib: os: cbprintf: Prevent using _Generic in C++

Add guard to not use C11 specific feature when in C++.

Upstream PR: zephyrproject-rtos/zephyr#34516

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
(cherry picked from commit eadf2fe8f4906d30400f6f8178c4593cac34852d)
Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>

include/sys/cbprintf.h

@@ -26,9 +26,9 @@ extern "C" {
  * - Clang 3.0 https://releases.llvm.org/3.0/docs/ClangReleaseNotes.html
  */
 #ifndef Z_C_GENERIC
-#if ((__STDC_VERSION__ >= 201112L) || \
+#if !defined(__cplusplus) && (((__STDC_VERSION__ >= 201112L) || \
 	((__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__) >= 40900) || \
-	((__clang_major__ * 10000 + __clang_minor__ * 100 + __clang_patchlevel__) >= 30000))
+	((__clang_major__ * 10000 + __clang_minor__ * 100 + __clang_patchlevel__) >= 30000)))
 #define Z_C_GENERIC 1
 #else
 #define Z_C_GENERIC 0