cmake: Enable specifying exact PowerPC CPU architecture

[ckastner]

Useful for when build host != use host. For example Debian's build daemons build packages for other hosts, so we want to target their CPUs, and not the build daemon's.

https://gcc.gnu.org/onlinedocs/gcc/RS_002f6000-and-PowerPC-Options.html#index-mcpu-10

[slaren]

For consistency, automatic CPU detection should be enabled with GGML_NATIVE:

if (GGML_NATIVE)
    # detect CPU
else()
    if (GGML_CPU_POWERPC_CPUTYPE)
        list(APPEND ARCH_FLAGS -mcpu=${GGML_CPU_POWERPC_CPUTYPE})
    endif()
endif()

[slaren]

Maybe we should consolidate all of these into a single option in the future, but it's not important right now.

[ckastner]

I'm happy to propose something once I've done more of a deep dive into other architectures. I'm not yet that familiar with the nuances of the other architectures, or how they would map. For example, -march vs. PowerPC's -mcpu and -mtune.

[slaren]

I am not sure either, but probably an option to set the value of -march or -mcpu would work well enough in most cases.