This commit moves the d8 Synopsis/Usage string from flags.cc into d8.cc.

The motivation for this is that Commit
c8679386 ("[snapshot] enable mksnapshot
usage to be displayed") enabled a usage string to be passed into

means mksnapshot --help will first print it's own usage string followed
by d8's.

FlagList: :SetFlagsFromCommandLine and be printed, but
FlagList: :PrintHelp currently still prints the Synopsis for d8 which
Change-Id: I9c79b2cdc5570106e12096c0c53e640cf80742e9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2299365Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68912}