deps: update V8 to 6.1

In progress because:

• SmartOS must be fixed properly upstream (ref)
• Waiting for the merge of https://github.com/v8/v8/commit/d8e37c32f359032f16c6b5d8d1b03ba99939e5cc in 6.1 (ref)
• 6.1 is still in beta

/cc @nodejs/platform-smartos @nodejs/v8

CI: https://ci.nodejs.org/job/node-test-commit/11673/

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

V8