FLAG_SCOPE_EXTERNAL can be used outside of the v8::internal namespace,
while FLAG_SCOPE can not.
This CL refactors FLAG_SCOPE such that it can be used anywhere, and
removes the redudant FLAG_SCOPE_EXTERNAL macro.
Also, UNFLAG_SCOPE_EXTERNAL is removed in favor of FLAG_SCOPE_VAL, as
the word "flag" in the macro name is meant to refer to a flag, and not
to the verb or action of "flagging" something, hence "unflag" does not
match the terminology.

R=ahaas@chromium.org

Bug: v8:11879
Change-Id: I2d761012c3e4330abb611bf67130eb57ec5bb964
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2988920
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75482}