Fix build on solaris platforms

e8b9f2d introduced a different cast which broke building on Illumos.
Revert to previous behavior for V8_OS_SOLARIS only.

Found on SmartOS while building with gcc 4.9.0.

edit1: adding jochen to reviewers since he was assigned through the issue tracker and danno since he seems to do a fair amount of cross-platform work

edit2: removing BUG reference because I don't understand what LOG needs to contain (and it seems to link to chromium and not the v8 repo). Please edit commit message as appropriate.

BUG=3935
LOG=n

Review URL: https://codereview.chromium.org/990063002

Cr-Commit-Position: refs/heads/master@{#27163}

AUTHORS

@@ -62,6 +62,7 @@ Jay Freeman <saurik@saurik.com>
 James Pike <g00gle@chilon.net>
 Jianghua Yang <jianghua.yjh@alibaba-inc.com>
 Joel Stanley <joel@jms.id.au>
+Johan Bergström <johan@bergstroem.nu>
 Jonathan Liu <net147@gmail.com>
 JunHo Seo <sejunho@gmail.com>
 Kang-Hao (Kenny) Lu <kennyluck@csail.mit.edu>

src/base/platform/platform-posix.cc

@@ -273,6 +273,8 @@ int OS::GetCurrentThreadId() {
   return static_cast<int>(gettid());
 #elif V8_OS_AIX
   return static_cast<int>(thread_self());
+#elif V8_OS_SOLARIS
+  return static_cast<int>(pthread_self());
 #else
   return static_cast<int>(reinterpret_cast<intptr_t>(pthread_self()));
 #endif