Add GN help for global switches.
Some of the more complicated ones like --args needed more explanation than could fit in one line. This separates out the switch help into a separate section accessed via "gn help switches".
Removes the switch --no-exec. Originally I used this to see the effect of running scripts on the build time, but now everything will fail hard all over the place if scripts don't return the expected output.
Review URL: https://codereview.chromium.org/
760933004
Cr-Commit-Position: refs/heads/master@{#305892}