Set V8_FATAL and V8_UNIMPLEMENTED to ((void) 0) in Release

Set V8_FATAL and V8_UNIMPLEMENTED to ((void) 0) in Release

V8_UNREACHABLE is already defined as ((void) 0) and all three macros are
already marked as unreliable in Release mode.

For example in loops the code is not optimized by the clang vectorizer
if a call to abort is possible.

BUG=

[skomski, 2015-09-25 08:14:13]

karl@skomski.com changed reviewers:
+ bmeurer@chromium.org

[Benedikt Meurer, 2015-09-25 08:41:48]

bmeurer@chromium.org changed reviewers:
+ jkummerow@chromium.org

[Benedikt Meurer, 2015-09-25 08:41:48]

bmeurer@chromium.org changed required reviewers:
+ jkummerow@chromium.org

[Benedikt Meurer, 2015-09-25 08:41:49]

Not sure if we want that. V8_FATAL and V8_UNIMPLEMENTED certainly help to track
certain Chrome crashes in the wild.

I'll forward that to the stability sheriff for decision.

[Jakob Kummerow, 2015-09-25 08:53:51]

Benedikt's guess is right, this does not lgtm. V8_FATAL in particular must crash
in Release mode.

If anything, we should consider making UNREACHABLE crash in Release mode too,
but I currently don't see any urgency to change anything.

If there are specific cases where we're missing out on important optimizations,
let's fix those places instead. (I'd be surprised if V8_UNIMPLEMENTED occurred
in hot loops.)