release: macOS 10.10 can't compile V8 6.8

[refack]

Refs: #1358 (comment)
Refs: #1409
Refs: nodejs/node#21079
Refs: #1264

V8 6.8 requires Xcode 8.x or higher but unfortunately Xcode 8.x is not supported on macOS 10.10

We only have a macOS 10.10 release machine, so ATM we can't release with V8 6.8.
The crux is the use on thread_local modifier for 2 variables used by torque which is a codegen tool used by the build process (no thread_local usage in the actual V8 code).
https://github.com/targos/node/blob/716948b9de79bbd327e9e496e4db8fc07e82b742/deps/v8/src/torque/contextual.h#L52-L57

We have a few options:

1. Upgrade the release machine
2. Patch the torque code (maybe as per https://stackoverflow.com/a/25393790/27955)
3. Cache the generated code (AFAICT it's identical when generated on macOS 10.10 and 10.11), and tweak the gyp files

/CC @nodejs/v8-update @nodejs/build-infra (specifical @targos and @rvagg)

[targos]

There is also option 4: add a new release machine.
See #1391

[refack]

Closing in favor of #1391