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: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15580>

src/amd/common/ac_gpu_info.c

@@ -701,8 +701,6 @@ bool ac_query_gpu_info(int fd, void *dev_p, struct radeon_info *info,
    /* Add some margin of error, though this shouldn't be needed in theory. */
    info->all_vram_visible = info->vram_size * 0.9 < info->vram_vis_size;
 
-   util_cpu_detect();
-
    /* Set chip identification. */
    info->pci_id = amdinfo->asic_id; /* TODO: is this correct? */
    info->pci_rev_id = amdinfo->pci_rev_id;

src/amd/compiler/tests/main.cpp

@@ -34,8 +34,6 @@
 #include "aco_ir.h"
 #include "framework.h"
 
-#include "util/u_cpu_detect.h"
-
 static const char *help_message =
    "Usage: %s [-h] [-l --list] [--no-check] [TEST [TEST ...]]\n"
    "\n"
@@ -243,8 +241,6 @@ int main(int argc, char **argv)
       return 99;
    }
 
-   util_cpu_detect();
-
    if (do_list) {
       for (auto test : tests)
          printf("%s\n", test.first.c_str());