Fix Musca-A1 gcc linker

[GaborAbonyi]

Summary of changes

Since 3e3af70, mbed_sdk_boot.c uses some linker definitions that are not defined on musca_a1 target with GCC_ARM compiler. This commit adds the missing definitions.

Impact of changes

It is possible again to compile mbed-os for musca_a1 target with GCC_ARM compiler.

Migration actions required

Documentation

N/A

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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

@tkaman

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[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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