toolchain/gcc: Limit use of _Static_assert

This C11 feature doesn't work when building C++ code, but the C++
version is based on C++11's static_assert(), which we don't have in
-std=c++98 either.

Signed-off-by: Andy Ross <andyross@google.com>

include/zephyr/toolchain/gcc.h

@@ -70,11 +70,14 @@
 #define BUILD_ASSERT(EXPR, MSG...) static_assert(EXPR, "" MSG)
 
 /*
- * GCC 4.6 and higher have the C11 _Static_assert built in, and its
+ * GCC 4.6 and higher have the C11 _Static_assert built in and its
  * output is easier to understand than the common BUILD_ASSERT macros.
+ * Don't use this in C++98 mode though (which we can hit, as
+ * static_assert() is not available)
  */
-#elif (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)) || \
-	(__STDC_VERSION__) >= 201100
+#elif !defined(CONFIG_STD_CPP98) && \
+	((__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)) ||	\
+	 (__STDC_VERSION__) >= 201100)
 #define BUILD_ASSERT(EXPR, MSG...) _Static_assert(EXPR, "" MSG)
 #else
 #define BUILD_ASSERT(EXPR, MSG...)