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: https://github.com/iojs/io.js/pull/1079


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>