STM Sleep change dependency func from static to WEAK

[hallard]

Summary of changes

change staticto WEAK for:

• ForcePeriphOutofDeepSleep
• ForceOscOutofDeepSleep
• wait_loop

Impact of changes

Migration actions required

Documentation

Since deep_sleep is declared as WEAK (perfect), I needed on STM32 to create my own deep_sleep because I need sleep to be STOP1 and not STOP2 (I'm using LPTIM2 in counter mode but that's another story)

But as deep_sleep calls ForcePeriphOutofDeepSleep, ForceOscOutofDeepSleep and wait_loop and as they are declared static they are unreachable from my code from link stage.

So I needed to duplicate them into my code (not my favorite) to get it working.
But best is to remove static so I can call them from my own deep_sleep (and put them also as WEAK to be consistent) without need to duplicate code. Also this will avoid to check for any changes on original code to replicate it on my own.

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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

@jeromecoutant @0xc0170

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[jeromecoutant]

Let's check CI result

[0xc0170]

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[mbed-ci]

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