- 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

Review URL: https://codereview.chromium.org/1310273009

Cr-Commit-Position: refs/heads/master@{#30640}