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}

Author: johan

AUTHORS

@@ -62,6 +62,7 @@ Jay Freeman <[email protected]>
 James Pike <[email protected]>
 Jianghua Yang <[email protected]>
 Joel Stanley <[email protected]>
+Johan Bergström <[email protected]>
 Jonathan Liu <[email protected]>
 JunHo Seo <[email protected]>
 Kang-Hao (Kenny) Lu <[email protected]>

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