This is a reland of b8f91666
Difference to previous CL: Additional functionality to specify
incompatible flags based on GN variables and extra-flags, used
to fix the issues that came up on the waterfall.

This also changes the rules regarding repeated flags: While
explicitly repeated flags are allowed for boolean values as long
as they are identical, repeated flags or explicit flags in the
presence of an active implication are disallowed for non-boolean
flags. The latter simplifies specifying conflict rules in
variants.py. Otherwise a rule like

INCOMPATIBLE_FLAGS_PER_EXTRA_FLAG = {
  "--gc-interval=*": ["--gc-interval=*"],
}

wouldn't work because specifying the same GC interval twice
wouldn't actually count as a conflict. This was an issue with
test/mjsunit/wasm/gc-buffer.js, which specifies
--gc-interval=500 exactly like the extra flag by the stress bot.

Also, this now expands contradictory flags checking to d8 flags
for consistency.

Original change's description:
> [flags] warn about contradictory flags
>
> Design Doc: https://docs.google.com/document/d/1lkvu8crkK7Ei39qjkPCFijpNyxWXsOktG9GB-7K34jM/
>
> Bug: v8:10577
> Change-Id: Ib9cfdffa401c48c895bf31caed5ee03545beddab
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2154792
> Reviewed-by: Clemens Backes <clemensb@chromium.org>
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Reviewed-by: Georg Neis <neis@chromium.org>
> Reviewed-by: Tamer Tas <tmrts@chromium.org>
> Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#68168}

Bug: v8:10577
Change-Id: I268e590ee18a535b13dee14eeb15ddd0a9ee8341
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2235115
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68989}