Add basic CI for -marm codegen using qemu

[dkorpel]

Add a dshell test for the Aarch64 code generation using qemu.

Module comment of arm_cross.d:

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Cross-compile AArch64 runnable tests and execute them under qemu-aarch64.

Can be run directly or via test runner:

rdmd compiler/test/dshell/arm_cross.d
compiler/test/run.d arm

Or via the test runner:
Install prerequisites:

Debian/Ubuntu:

sudo apt install qemu-user clang lld gcc-aarch64-linux-gnu

Arch Linux:

sudo pacman -S qemu-user clang lld aarch64-linux-gnu-gcc

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Currently these files pass:

runnable/ai
runnable/arm
runnable/bcraii
runnable/bcraii2
runnable/opcolon
runnable/powinline
runnable/test18472
runnable/test21416
runnable/test24884

Eventually, this list should be inverted from 'known successes' to 'known failures', and the test logic should be integrated in d_do_test.d. At this early stage however, it's better to have a separate test script.

[dlang-bot]

Thanks for your pull request and interest in making D better, @dkorpel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• 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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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 "master + dmd#23155"