Add s390x support to GHA workflow

[Darragh-Drohan]

Hi,

This PR adds support for s390x to the GitHub Actions workflow. llvmdev builds successfully and the results of the LLVM tests were:

Total Discovered Tests: 16827
Excluded : 1 (0.01%)
Unsupported : 404 (2.40%)
Passed : 16362 (97.24%)
Expectedly Failed: 60 (0.36%)

llvmlite also builds successfully and the results of the tests were:

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GitHub does not host an s390x runner, however open source projects can request access to the IBM hosted s390x/ppc64le runners by submitting an issue on the ActionsPZ GitHub repo.

The process is straight forward:

1. An admin submits an issue on the actionsPZ repo using this template
2. The same admin creates an IBM ID using the same email as their GitHub account.
3. Once approved the GitHub app can be added to the project which allows the use of s390x and POWER9/10 runners.

We are happy to make any changes needed to this PR and provide support when requesting access to actionspz.

Ref: #780

[esc]

@Darragh-Drohan thank you very much for opening this! The maintainers will discuss this during an upcoming meeting and we will get back to you.

[esc]

@Darragh-Drohan I have updated the s390x support ticket here: #780 (comment)