[nrf toup] Do not use GNU strerror while using Zephyr

In some cases GNU_SOURCE may be enabled and due to that
a wrong strerror implementation is used for Zephyr platform.
To prevent this, exclude ZEPHYR if GNU_SOURCE is enabled.

Signed-off-by: Arkadiusz Balys <arkadiusz.balys@nordicsemi.no>

Author: ArekBalysNordic

src/system/SystemError.cpp

@@ -91,7 +91,7 @@ DLL_EXPORT const char * DescribeErrorPOSIX(CHIP_ERROR aError)
 #endif // CHIP_SYSTEM_CONFIG_THREAD_LOCAL_STORAGE
 
     // Use thread-safe strerror_r when available
-#if defined(_GNU_SOURCE) && !defined(__ANDROID__) && !defined(__Fuchsia__) && !defined(__MUSL__)
+#if defined(_GNU_SOURCE) && !defined(__ANDROID__) && !defined(__Fuchsia__) && !defined(__MUSL__) && !defined(__ZEPHYR__)
     // GNU version returns char*
     const char * s = strerror_r(lError, errBuf, sizeof(errBuf));
     if (s != nullptr)