Fix building alpaka examples

[bernhardmgruber]

Due to an upstream rename of alpaka's cmake variables (alpaka-group/alpaka#1653), LLAMA's cmake did not correctly detect if alpaka was found, and did not build any alpaka examples. Thus, some modifications in LLAMA did not propagate to the alpaka examples, and they needed fixing.

As a side change in this PR, the index type used in the asynccopy example was changed from size_t to int.

[codecov]

Codecov Report

Merging #527 (ccbc3cf) into develop (c77415a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head ccbc3cf differs from pull request most recent head 2a36ee9. Consider uploading reports for the commit 2a36ee9 to get more accurate results

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##           develop     #527   +/-   ##
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  Coverage    99.25%   99.25%           
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  Files           63       63           
  Lines         5508     5510    +2     
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+ Hits          5467     5469    +2     
  Misses          41       41