Fix platform specific parameters for ppc64le

[oturpe]

Description

Previously, build script assumed that for little-endian PowerPC, Meson would report ppc64le as cpu_family. Actually, Meson returns just ppc64 and endianness needs to be checked separately. Fix the Meson build script to do that.

Without this change, compiling on ppc64le included 32-bit PowerPC code from risc_ppc.h, causing many error about invalid casts from pointer to 32-bit integer types.

This issue was revealed by building DOSBox Staging 0.82.0 for Fedora in Fedora build environment. Builds for other architectures (x86_64, aarch64 and s390x) succeeded, but ppc64le failed due to this problem.

Related issues

#3991

Manual testing

After applying this patch, Fedora build succeeded for all architectures (x86_64, aarch64, ppc64le and s390x). The one for which I have hardware for, x86_64, also ran successfully.

Checklist

I have:

• [x] followed the project's contributing guidelines and code of conduct.
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• [ ] confirmed that my code does not cause performance regressions (e.g., by running the Quake benchmark).
• [ ] added unit tests where applicable to prove the correctness of my code and to avoid future regressions.
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