refactor: call CPU backend functions dynamically

[wbruna]

Summary

Replaces ggml_backend_cpu_init, ggml_backend_is_cpu, ggml_backend_cpu_set_n_threads functions with dynamic ggml backend calls, and avoids ggml_cpu_has_* by getting the flags with a function provided by ggml.

These functions are not available without native CPU builds. With these changes, I was able to build and run sd.cpp with a runtime-selected CPU backend (-DSD_BUILD_SHARED_LIBS=ON -DSD_BUILD_SHARED_GGML_LIB=ON -DGGML_BACKEND_DL=ON -DGGML_CPU_ALL_VARIANTS=ON).

It changes the CPU info log, since ggml only reports the active flags, but I've tried to keep the formatting similar:

[DEBUG] main.cpp:598 - System Info:
SSE3 = 1 | SSSE3 = 1 | AVX = 1 | AVX2 = 1 | F16C = 1 | FMA = 1 | BMI2 = 1 | OPENMP = 1 | REPACK = 1 |

Related Issue / Discussion

Should fix #1567, although the issue with GGML_MAX_NAME would remain.

Could help with #1589, #1588, #1483, #1351 by allowing binary releases using dynamically-selected ggml CPU backends.

Complements the runtime backend selection (#1448).

Checklist

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