Fix for missing os_cb_section

[rajkan01]

-Added the os_cb_section stub to remove the warnings

Description

Procedure to reproduce the issue:
1.Clone the blinky bare metal from https://github.com/ARMmbed/mbed-os-example-blinky-baremetal.git
2. Compile the blinky bare metal with "mbed compile -t ARMC6 -m target --profile release"
3.End of the compilation you will notice "[Warning] @0,0: L6320W: Ignoring --keep command. Cannot find argument 'os_cb_sections'." warning.

Reason for the issue:

1. In release profile, ARMC6 compiler have extra optional argument --keep=os_cb_sections but os_cb_section defined inside RTX RTOS(refer mbed-os/rtos)
   2.For bare metal build we dont have RTX RTOS.

Fix for the issue:

1. Added the osc_cb_section stub to remove the warnings.

This fixes #11565

Pull request type

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

Reviewers

@evedon @hugueskamba

Release Notes

[ciarmcom]

@rajkan01, thank you for your changes.
@ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[hugueskamba]

@rajkan01
Please add the following text to the description so the associated GitHub issue can be automatically closed when this is merged.

This fixes #11565

It will also add a reference of this PR in the GitHub issue.

Could you also provide the steps to reproduce the error before we try your fix?

[0xc0170]

Travis astyle failed (one space left), please fix

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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