CMake: Fix Microlib selection

[hugueskamba]

Summary of changes

Fixes #13858

As the assembler does not seem to recognise the request to the
linker to use Microlib, remove that instruction.

Microlib is still used as the linker selects microlib as a result
of being explicitly asked to do so.

mbed-os-example-blinky with Microlib

Total Static RAM memory (data + bss): 9305(+9305) bytes
Total Flash memory (text + data): 35711(+35711) bytes

mbed-os-example-blinky with ARM standard C library

Total Static RAM memory (data + bss): 10374(+10374) bytes
Total Flash memory (text + data): 40482(+40482) bytes

Microlib selection documentation:
https://developer.arm.com/documentation/dui0808/e/the-arm-c-micro-library/building-an-application-with-microlib

Impact of changes

Migration actions required

Documentation

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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

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[ciarmcom]

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

[0xc0170]

Use --library_type=microlib with the linker to override all other options.

👍

[MarceloSalazar]

Works like a charm :)