util: Remove util_cpu_detect

util_cpu_detect is an anti-pattern: it relies on callers high up in the call
chain initializing a local implementation detail. As a real example, I added:

...a Mali compiler unit test
...that called bi_imm_f16() to construct an FP16 immediate
...that calls _mesa_float_to_half internally
...that calls util_get_cpu_caps internally, but only on x86_64!
...that relies on util_cpu_detect having been called before.

As a consequence, this unit test:

...crashes on x86_64 with USE_X86_64_ASM set
...passes on every other architecture
...works on my local arm64 workstation and on my test board
...failed CI which runs on x86_64
...needed to have a random util_cpu_detect() call sprinkled in.

This is a bad design decision. It pollutes the tree with magic, it causes
mysterious CI failures especially for non-x86_64 developers, and it is not
justified by a micro-optimization.

Instead, let's call util_cpu_detect directly from util_get_cpu_caps, avoiding
the footgun where it fails to be called.  This cleans up Mesa's design,
simplifies the tree, and avoids a class of a (possibly platform-specific)
failures. To mitigate the added overhead, wrap it all in a (fast) atomic
load check and declare the whole thing as ATTRIBUTE_CONST so the
compiler will CSE calls to util_cpu_detect.

Co-authored-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Part-of: <!15580>
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