Add Arduino Giga and Portenta support

[multiplemonomials]

Summary of changes

This PR updates Mbed OS to support the Arduino Giga and Portenta. Quite a number of changes for these targets and the broader Arduino framework have already been integrated through other PRs -- this just adds working target definitions and makes some final tweaks!

Impact of changes

Arduino Giga and Arduino Portenta (CM7 cores at least) are now supported as Mbed OS targets! This includes wifi support on both boards, and Ethernet support on the Portenta.

Note that the Portenta was theoretically already supported, but didn't actually build, and didn't have any upload methods. Now it's properly supported.

Also note that both boards require an external STLINK-V3SET for debugging, but support flashing via a DFU bootloader.

Migration actions required

Documentation

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[X] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

These boards use STM32H7 MCUs which have already been pretty extensively tested with Mbed OS. I have personally tested wifi and Ethernet on both boards and things seem to be OK. Unfortunately there's no automated testing in the Arduino framework...

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