Implement GGML_CPU_ALL_VARIANTS for ARM (llama/14080)

Implement GGML_CPU_ALL_VARIANTS for ARM (llama/14080)

* ggml-cpu: Factor out feature detection build from x86

* ggml-cpu: Add ARM feature detection and scoring

This is analogous to cpu-feats-x86.cpp. However, to detect compile-time
activation of features, we rely on GGML_USE_<FEAT> which need to be set
in cmake, instead of GGML_<FEAT> that users would set for x86.

This is because on ARM, users specify features with GGML_CPU_ARM_ARCH,
rather than with individual flags.

* ggml-cpu: Implement GGML_CPU_ALL_VARIANTS for ARM

Like x86, however to pass around arch flags within cmake, we use
GGML_INTERNAL_<FEAT> as we don't have GGML_<FEAT>.

Some features are optional, so we may need to build multiple backends
per arch version (armv8.2_1, armv8.2_2, ...), and let the scoring
function sort out which one can be used.

* ggml-cpu: Limit ARM GGML_CPU_ALL_VARIANTS to Linux for now

The other platforms will need their own specific variants.

This also fixes the bug that the the variant-building branch was always
being executed as the else-branch of GGML_NATIVE=OFF. The branch is
moved to an elseif-branch which restores the previous behavior.