v8: back-port openbsd/amd64 build fix

Cherry-pick https://codereview.chromium.org/856553002 from upstream.

Makes V8 on OpenBSD/amd64 pick up the right V8_INT64_C and V8_UINT64_C
macros.

PR-URL: https://github.com/iojs/io.js/pull/1318
Refs: https://github.com/iojs/io.js/pull/1312


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>