cmake: skip x86 cpu-feats helper in hybrid GGML_BACKEND_DL + GGML_CPU_STATIC builds

The cpu-feats OBJECT library implements the DL loader's variant score; it
is meaningful only when the CPU backend itself is a dlopen'd module. In
hybrid mode (GGML_BACKEND_DL=ON + GGML_CPU_STATIC=ON - the qvac diffusion
port configuration) the CPU backend is a static library registered
directly in-process, and PRIVATE-linking the un-exported OBJECT helper
into it makes configure fail at install(EXPORT ggml-targets):
  includes target "ggml-cpu" which requires target "ggml-cpu-feats"
  that is not in any export set.
ARM never hit this because its feats call is gated on
GGML_CPU_ALL_VARIANTS; x86 gated it on GGML_BACKEND_DL alone. Needed to
extend the Android hybrid mode to desktop Linux (QVAC-23767 / qvac#3853).

Author: DmitryMalishev

src/ggml-cpu/CMakeLists.txt

@@ -387,7 +387,15 @@ function(ggml_add_cpu_backend_variant_impl tag_name)
             endif()
         endif()
 
-        if (GGML_BACKEND_DL)
+        # The cpu-feats score object only matters when the CPU backend is a
+        # dlopen'd module that must score itself against the host CPU. In
+        # hybrid mode (GGML_BACKEND_DL + GGML_CPU_STATIC) the CPU backend is a
+        # static library registered directly in-process, the score is never
+        # consulted, and the OBJECT helper breaks install(EXPORT ggml-targets)
+        # ("requires target ggml-cpu-feats that is not in any export set").
+        # ARM already skips it implicitly (feats is only added under
+        # GGML_CPU_ALL_VARIANTS there).
+        if (GGML_BACKEND_DL AND NOT GGML_CPU_STATIC)
             if (GGML_NATIVE)
                 # the feature check relies on ARCH_DEFINITIONS, but it is not set with GGML_NATIVE
                 message(FATAL_ERROR "GGML_NATIVE is not compatible with GGML_BACKEND_DL, consider using GGML_CPU_ALL_VARIANTS")