v8: fix build on solaris platforms

`v8/3c7e4403` introduced a different cast which broke building on
Illumos. Revert to previous behavior for V8_OS_SOLARIS. Found on
SmartOS while building with gcc 4.9.0.

V8-Issue: https://code.google.com/p/v8/issues/detail?id=3935
V8-Patch: https://codereview.chromium.org/990063002
PR-URL: #1079
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

deps/v8/src/base/platform/platform-posix.cc

@@ -260,6 +260,8 @@ int OS::GetCurrentThreadId() {
   return static_cast<int>(syscall(__NR_gettid));
 #elif V8_OS_ANDROID
   return static_cast<int>(gettid());
+#elif V8_OS_SOLARIS
+  return static_cast<int>(pthread_self());
 #else
   return static_cast<int>(reinterpret_cast<intptr_t>(pthread_self()));
 #endif