Replace __attribute__((nonstring)) with macro MBEDTLS_ATTRIBUTE_UNTERMINATED_STRING
This macro applies __attribute__((nonstring)) when using a compiler that supports it

Upstream-Status: Backport [https://github.com/Mbed-TLS/TF-PSA-Crypto/commit/996f4fa3a2fbe8792ed3efd1bcb3657001f35ae1]

Signed-off-by: Felix Conway <felix.conway@arm.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

--- a/library/ssl_tls13_keys.h
+++ b/library/ssl_tls13_keys.h
@@ -7,6 +7,22 @@
 #if !defined(MBEDTLS_SSL_TLS1_3_KEYS_H)
 #define MBEDTLS_SSL_TLS1_3_KEYS_H
 
+/* GCC >= 15 has a warning 'unterminated-string-initialization' which complains if you initialize
+ * a string into an array without space for a terminating NULL character. In some places in the
+ * codebase this behaviour is intended, so we add the macro MBEDTLS_ATTRIBUTE_UNTERMINATED_STRING
+ * to suppress the warning in these places.
+ */
+#if defined(__has_attribute)
+#if __has_attribute(nonstring)
+#define MBEDTLS_HAS_ATTRIBUTE_NONSTRING
+#endif /* __has_attribute(nonstring) */
+#endif /* __has_attribute */
+#if defined(MBEDTLS_HAS_ATTRIBUTE_NONSTRING)
+#define MBEDTLS_ATTRIBUTE_UNTERMINATED_STRING __attribute__((nonstring))
+#else
+#define MBEDTLS_ATTRIBUTE_UNTERMINATED_STRING
+#endif /* MBEDTLS_HAS_ATTRIBUTE_NONSTRING */
+
 /* This requires MBEDTLS_SSL_TLS1_3_LABEL( idx, name, string ) to be defined at
  * the point of use. See e.g. the definition of mbedtls_ssl_tls13_labels_union
  * below. */