toolchain: Add flag indicating zero length array support

Add flag which indicates if a toolchain supports ZLA (Zero
Length Arrays). It is a GCCC extension inherited by clang as
well.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>

include/zephyr/toolchain.h

@@ -115,6 +115,14 @@
 #define TOOLCHAIN_HAS_C_AUTO_TYPE 0
 #endif
 
+/**
+ * @def TOOLCHAIN_HAS_ZLA
+ * @brief Indicate if toolchain supports Zero Length Arrays.
+ */
+#ifndef TOOLCHAIN_HAS_ZLA
+#define TOOLCHAIN_HAS_ZLA 0
+#endif
+
 /**
  * @def TOOLCHAIN_IGNORE_WSHADOW_BEGIN
  * @brief Begin of block to ignore -Wshadow.

include/zephyr/toolchain/gcc.h

@@ -30,6 +30,8 @@
 #define TOOLCHAIN_HAS_C_AUTO_TYPE 1
 #endif
 
+#define TOOLCHAIN_HAS_ZLA 1
+
 /*
  * Older versions of GCC do not define __BYTE_ORDER__, so it must be manually
  * detected and defined using arch-specific definitions.