Move rtos & platform TARGET directories to source

[gpsimenos]

Description

As a follow-up to the Public Headers PR, we agreed to:

• Move rtos/TARGET_CORTEX to rtos/source/TARGET_CORTEX
• Move platform/TARGET_CORTEX_M to platform/source/TARGET_CORTEX_M

Pull request type

[ ] Fix
[X] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

Release Notes

[0xc0170]

Please also update astyleignore file (3rd party files are now being checked, rtos)

[mbed-ci]

Test run: FAILED

Summary: 3 of 12 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_pdmc-test
• jenkins-ci/mbed-os-ci_exporter
• jenkins-ci/mbed-os-ci_dynamic-memory-usage

[0xc0170]

Ci restarted (couple of internal errors fixed recently)

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 2
Build artifacts