Don't run macros or jsmin on extra snapshot scripts

Don't run macros or jsmin on extra snapshot scripts

js2c.py now distinguishes between the JS internal sources and any
extra scripts passed in. The latter only get validation and
comment/trailing whitespace removal applied now.

Also added a --js option to js2c.py, which will output a JS file
instead of a C++ file, for debugging. (I got tired of finding the
generated .cc file and extracting its byte array so that I could feed
it to a separate helper script I wrote.)

R=yangguo@chromium.org, jochen@chromium.org
BUG=v8:4064
LOG=N

Review URL: https://codereview.chromium.org/1117663002

Cr-Commit-Position: refs/heads/master@{#28133}