Fix CMake dependency error with custom linker scripts

[multiplemonomials]

Summary of changes

Fix dependency error with custom linker scripts. Back in #362 I added the LINK_DEPENDS property so that executables would be rebuilt if the linker script changed. However, it caused a circular dependency error with custom linker scripts, because the linker script was expected to exist before the link command (which generated the linker script) could be run! This PR fixes this by putting the dependency on the raw file instead of the preprocessed one.

Impact of changes

Migration actions required

Documentation

None

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)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

This can build @JojoS62 's test project for custom linker scripts

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[JojoS62]

my older project works with these changes again.