Cleanup V8_TURBOFAN_TARGET definition.

Cleanup V8_TURBOFAN_TARGET definition.

Move to globals.h so we do no longer skip any number of
tests. Introduce V8_TURBOFAN_BACKEND, which is 1 if we
actually have a backend and want to compile it, but don't
run the tests.

R=titzer@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=22863

[Benedikt Meurer, 2014-08-05 11:51:03]

PTAL

[titzer, 2014-08-05 11:53:06]

lgtm

[Benedikt Meurer, 2014-08-05 11:53:42]

Committed patchset #1 manually as 22863 (presubmit successful).