[build] Disable "control reaches end of non-void function" warning on GCC builds

R=machenbach@chromium.org, thakis@chromium.org

Bug: v8:7658
Change-Id: Ib1ee0c2efb9eca9e3c9c5012ea69b5df4e831be9
Reviewed-on: https://chromium-review.googlesource.com/1021811
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52712}

BUILD.gn

@@ -574,6 +574,11 @@ config("toolchain") {
       "/wd4703",  # Potentially uninitialized local pointer variable.
       "/wd4709",  # Comma operator within array index expr (bugged).
       "/wd4714",  # Function marked forceinline not inlined.
+
+      # MSVC assumes that control can get past an exhaustive switch and then
+      # warns if there's no return there (see https://crbug.com/v8/7658)
+      "/wd4715",  # Not all control paths return a value.
+
       "/wd4718",  # Recursive call has no side-effect.
       "/wd4800",  # Forcing value to bool.
     ]
@@ -585,6 +590,10 @@ config("toolchain") {
       # signed overflow does not occur. Generates false positives (see
       # http://crbug.com/v8/6341).
       "-Wno-strict-overflow",
+
+      # GCC assumes that control can get past an exhaustive switch and then
+      # warns if there's no return there (see https://crbug.com/v8/7658).
+      "-Wno-return-type",
     ]
   }
 }