Only test druntime exceptions gdb test case (rt_trap_exceptions_drt_g…

[JohanEngelen]

…db) when gdb is available.

Fixes CI systems where gdb is not available (LDC)

[dlang-bot]

Thanks for your pull request and interest in making D better, @JohanEngelen! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + dmd#16513"