druntime: Update GNU implementation of callWithStackShell

[ibuclaw]

The generic solution using a call to __builtin_unwind_init () followed by an assignment to sp has two issues:

1. On some archs it stores a huge amount of FP and Vector state which is not the subject of the scan - and, indeed might produce false hits.
2. Even on archs like X86, where there are no callee-saved FPRs/VRs there tend to be 'holes' in the frame allocations (to deal with alignment) which also will contain random data which could produce false positives.

This solution stores only the integer callee-saved registers.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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dub run digger -- build "master + dmd#20736"