Fix build on solaris platforms
e8b9f2d7 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.