[toolchain] suppress gcc string operation warning (#11810)

Introduces macros to suppress a known false-positive GCC warning
"-Wstringop-overflow=0" which can be triggered when manipulating
network data.

The `AddHasRoute()`, `AddBorderRouter()`, and `AddServer()` methods
shift and update the network data bytes, which may involve inserting
or updating a sub-TLV within an existing TLV. This can trigger a
"writing x byte into a region of size 0" error on some GCC
toolchains.

This change adds the `OT_SUPPRESS_GCC_STRING_OP_BEGIN` and
`OT_SUPPRESS_GCC_STRING_OP_END` macros to silence this specific
warning within these code blocks.

include/openthread/instance.h

@@ -52,7 +52,7 @@ extern "C" {
  *
  * @note This number versions both OpenThread platform and user APIs.
  */
-#define OPENTHREAD_API_VERSION (526)
+#define OPENTHREAD_API_VERSION (527)
 
 /**
  * @addtogroup api-instance

include/openthread/platform/toolchain.h

@@ -293,6 +293,23 @@ extern "C" {
     } while (false) /* fallthrough */
 #endif
 
+// A known false positive warning occurs on some GCC toolchains,
+// resulting in "error: writing x byte into a region of size 0". The following
+// macros are used to suppress this warning/error in specific code blocks.
+
+#if defined(__GNUC__) && (__GNUC__ >= 7)
+
+#define OT_SUPPRESS_GCC_STRING_OP_BEGIN \
+    _Pragma("GCC diagnostic push") _Pragma("GCC diagnostic warning \"-Wstringop-overflow=0\"")
+#define OT_SUPPRESS_GCC_STRING_OP_END _Pragma("GCC diagnostic pop")
+
+#else
+
+#define OT_SUPPRESS_GCC_STRING_OP_BEGIN
+#define OT_SUPPRESS_GCC_STRING_OP_END
+
+#endif
+
 /**
  * @}
  */

src/core/thread/network_data_leader_ftd.cpp

@@ -835,6 +835,15 @@ exit:
     return error;
 }
 
+// The `AddHasRoute()`, `AddBorderRouter()`, and `AddServer()` methods
+// below shift and update the network data bytes, which may involve
+// inserting or updating a sub-TLV within an existing TLV. This can
+// trigger a known false positive warning on some GCC toolchains. The
+// `OT_SUPPRESS_GCC_STRING_OP_BEGIN`/ `OT_SUPPRESS_GCC_STRING_OP_END`
+// macros are used to silence this check within these methods.
+
+OT_SUPPRESS_GCC_STRING_OP_BEGIN
+
 Error Leader::AddHasRoute(const HasRouteTlv &aHasRoute, PrefixTlv &aDstPrefix, ChangedFlags &aChangedFlags)
 {
     Error                error       = kErrorNone;
@@ -969,6 +978,8 @@ exit:
     return error;
 }
 
+OT_SUPPRESS_GCC_STRING_OP_END
+
 Error Leader::AllocateServiceId(uint8_t &aServiceId) const
 {
     Error   error = kErrorNotFound;