Fix ARM compiler warning for microlib

[rajkan01]

Summary of changes

• if the "target.c_lib": "small" is configured in the build tool, then ARM compiler issues a warning as [DEBUG] Output: armclang: warning: -Wl,--library_type=microlib: 'linker' input unused [-Wunused-command-line-argument] .
• The reported issue is solved by adding that flag only in the linker configuration as per docs.

Impact of changes

With these changes, compiler warning about unused command-line argument should go away.

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

@evedon

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[ciarmcom]

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

[adbridge]

The reported issue is solved by adding that flag only in the linker configuration as per docs.

And is this flag in the linker configuration ?

[adbridge]

CI started

[adbridge]

@rajkan01 looks like you have some genuine errors. For ARCHPRO for example:
[2020-01-24T11:09:04.227Z] Compile [ 6.6%]: USBCDC_ECM.cpp
[2020-01-24T11:09:04.227Z] [ERROR] ./drivers/source/usb/USBCDC_ECM.cpp:558:1: internal compiler error: in output_die, at dwarf2out.c:10807
[2020-01-24T11:09:04.227Z] 558 | }
[2020-01-24T11:09:04.227Z] | ^

[rajkan01]

@rajkan01 looks like you have some genuine errors. For ARCHPRO for example:
[2020-01-24T11:09:04.227Z] Compile [ 6.6%]: USBCDC_ECM.cpp
[2020-01-24T11:09:04.227Z] [ERROR] ./drivers/source/usb/USBCDC_ECM.cpp:558:1: internal compiler error: in output_die, at dwarf2out.c:10807
[2020-01-24T11:09:04.227Z] 558 | }
[2020-01-24T11:09:04.227Z] | ^

This PR changes related to ARM toolchain with microlib library but failure in GCC_ARM. Please restart the CI.

[mbed-ci]

Test run: FAILED

Summary: 3 of 4 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_build-GCC_ARM
• jenkins-ci/mbed-os-ci_build-IAR
• jenkins-ci/mbed-os-ci_build-ARM

[adbridge]

CI restarted

[mbed-ci]

Test run: SUCCESS

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

[mergify]

This PR does not contain release version label after merging.

[0xc0170]

This PR does not contain release version label after merging.

I'll look at this shortly. It should be fine as it has release version