Remove all gyp BUILD rules with multiple outputs.

Remove all gyp BUILD rules with multiple outputs.

- Modify js2c to accept --js and --nojs,
- modify mksnapshot to accept --startup_src
  (instead of a positional parameter, so that it can be omitted),
- modify v8.gyp to use the above so that no target has multiple
  output dependencies, and
- update GN to use the switches above.

(I have not succeeded in fixing the GYP->make translator to properly map
 multi-output rules, so that they work as expected in all edge cases.
 This CL signals defeat on that front, and instead I rewrite the GYP
 file to avoid that situation in the first place.)

R=jochen@chromium.org
BUG=v8:4382
LOG=N

Committed: https://crrev.com/244cc0a6a9643bee962c2e3c60a1e093f18fe9ed
Cr-Commit-Position: refs/heads/master@{#30640}

[vogelheim, 2015-09-08 14:05:20]

Please let me know whether the approach is acceptable. I have to admit defeat on
the GYP front.

[jochen (gone - plz use gerrit), 2015-09-08 14:12:08]

lgtm

[vogelheim, 2015-09-08 15:23:04]

The CQ bit was checked by vogelheim@chromium.org

[commit-bot: I haz the power, 2015-09-08 15:23:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1310273009/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1310273009/1

[commit-bot: I haz the power, 2015-09-08 15:24:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-08 15:25:04]

Patchset 1 (id:??) landed as
https://crrev.com/244cc0a6a9643bee962c2e3c60a1e093f18fe9ed
Cr-Commit-Position: refs/heads/master@{#30640}