Fix stub linker error (youtube#5558)

b/413130400

Add warning override as the compiler correctly detects code which is
unreachable as COBALT_LINKER_STUB is supposed to crash in devel builds.

Ensure that the stub method doesnt trigger a crash in prod builds.
Add include gaurds to the linker_stub.h header file

Author: niranjanyardi

base/BUILD.gn

@@ -1760,6 +1760,7 @@ component("base") {
       "process/process_metrics_posix_starboard_stub.cc",
       "starboard/linker_stub.h"
     ]
+    all_dependent_configs += [ ":base_starboard_linker_stub" ]
   }
 
   if (is_cobalt && enable_cobalt_hermetic_hacks) {
@@ -5033,3 +5034,11 @@ source_set("partition_alloc_test_support") {
     configs += [ "//build/config/compiler:optimize_speed" ]
   }
 }
+
+if (is_cobalt_hermetic_build) {
+  # TODO: b/413130400 - Cobalt: Remove this config 
+  # after removing base/starboard/linker_stub.h
+  config("base_starboard_linker_stub") {
+    cflags = ["-Wno-unreachable-code-break"]
+  }
+}

base/starboard/linker_stub.h

@@ -12,10 +12,16 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#ifndef BASE_STARBOARD_LINKER_STUB_H_
+#define BASE_STARBOARD_LINKER_STUB_H_
+
 #include "starboard/common/log.h"
 
 // TODO: b/413130400 - Cobalt: Remove this file and implement linker stubs
 
+#if BUILDFLAG(COBALT_IS_RELEASE_BUILD)
+#define COBALT_LINKER_STUB()
+#else
 #define COBALT_LINKER_STUB_MSG                                    \
   "This is a stub needed for linking cobalt. You need to remove " \
   "COBALT_LINKER_STUB and implement "                             \
@@ -29,3 +35,6 @@
     }                                          \
   } while (0);                                 \
   SbSystemBreakIntoDebugger();
+#endif  // BUILDFLAG(COBALT_IS_RELEASE_BUILD)
+
+#endif  // BASE_STARBOARD_LINKER_STUB_H_