C++11-ify virtualisation in FileHandle + Serials

[kjbracey]

Summary of changes

Use override and final where appropriate, and eliminate unnecessary virtual.

Impact of changes

api_lock in BufferedSerial is no longer virtual, unlike UARTSerial. This shouldn't affect applications, as there's no known use for overriding this, unlike the lock methods in basic drivers.

Slight code size reduction.

Migration actions required

None.

Documentation

None

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] 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)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@hugueskamba

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[ciarmcom]

@kjbracey-arm, thank you for your changes.
@hugueskamba @ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts