Fix warnings from kill and getpid not being implemented

[multiplemonomials]

Summary of changes

Versions of GCC which supply newer Newlib versions (unsure of the exact Newlib version, I think it's the one included with GCC 11 and newer) now appear to want the _getpid() and _kill() functions defined. I checked using radare, and it appears they get pulled in as dependencies of the C++ standard library, which has been built with exception handling enabled and therefore calls things like abort(). So, these warnings will show up whenever someone links an application which uses STL features.

In the long term the best solution is to rebuild the STL with exceptions disabled, or switch to a toolchain which has that done for us, but in the meantime this will stop the warnings from bothering people.

Impact of changes

None

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

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