docs/system/arm: Document which architecture extensions we emulate

docs/system/arm: Document which architecture extensions we emulate

These days the Arm architecture has a wide range of fine-grained
optional extra architectural features. We implement quite a lot
of these but by no means all of them. Document what we do implement,
so that users can find out without having to dig through back-issues
of our Changelog on the wiki.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20210617140328.28622-1-peter.maydell@linaro.org
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>