Another -Wunreachable-code-aggressive error.

Bug: chromium:1066980
Change-Id: I189e208e9d089967bfa1b4f27ffdda49938a1f5f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3019184
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75668}

src/base/platform/platform-posix.cc

@@ -1051,8 +1051,9 @@ Stack::StackSlot Stack::GetStackStart() {
   // the start of the stack.
   // See https://code.google.com/p/nativeclient/issues/detail?id=3431.
   return __libc_stack_end;
-#endif  // !defined(V8_LIBC_GLIBC)
+#else
   return nullptr;
+#endif  // !defined(V8_LIBC_GLIBC)
 }
 
 #endif  // !defined(V8_OS_FREEBSD) && !defined(V8_OS_MACOSX) &&