Enable generic clang backend tests.

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### Description
<!--Please explain your changes.-->

Enable generic clang backend tests.

Added an AVX512 job using the generic clang backend.

Also fixed up some guards in the custom AVX512 gemm/trsm kernels so they don't
start defining things if they aren't used.

See merge request libeigen/eigen!2063

ci/build.linux.gitlab-ci.yml

@@ -97,6 +97,18 @@ build:linux:cross:x86-64:clang-12:avx512dq:
   variables:
     EIGEN_CI_ADDITIONAL_ARGS: "-DEIGEN_TEST_AVX512DQ=on"
 
+# Generic vector extension backend.
+build:linux:cross:x86-64:clang-19:generic:avx512dq:
+  image: ubuntu:24.04
+  extends: build:linux:cross:x86-64:clang-12:default
+  variables:
+    EIGEN_CI_INSTALL: clang-19
+    EIGEN_CI_C_COMPILER: clang-19
+    EIGEN_CI_CXX_COMPILER: clang++-19
+    EIGEN_CI_CROSS_INSTALL: g++-14-x86-64-linux-gnu clang-19
+    EIGEN_CI_ADDITIONAL_ARGS: >
+      -DEIGEN_TEST_CUSTOM_CXX_FLAGS=-mfma;-mavx512dq;-DEIGEN_VECTORIZE_GENERIC=1
+
 build:linux:docs:
   extends: .build:linux:cross
   variables:

ci/test.linux.gitlab-ci.yml

@@ -197,6 +197,29 @@ test:linux:x86-64:clang-12:avx512dq:unsupported:
   variables:
     EIGEN_CI_CTEST_LABEL: Unsupported
 
+# Generic vector extension backend.
+.test:linux:x86-64:clang-19:generic:avx512dq:
+  image: ubuntu:24.04
+  extends: .test:linux:x86-64
+  needs: [ build:linux:cross:x86-64:clang-19:generic:avx512dq ]
+  variables:
+    EIGEN_CI_INSTALL: clang-19
+  tags:
+    - eigen-runner
+    - linux
+    - x86-64
+    - avx512
+
+test:linux:x86-64:clang-19:generic:avx512dq:official:
+  extends: .test:linux:x86-64:clang-19:generic:avx512dq
+  variables:
+    EIGEN_CI_CTEST_LABEL: Official
+
+test:linux:x86-64:clang-19:generic:avx512dq:unsupported:
+  extends: .test:linux:x86-64:clang-19:generic:avx512dq
+  variables:
+    EIGEN_CI_CTEST_LABEL: Unsupported
+
 ##### CUDA #####################################################################
 .test:linux:cuda:
   extends: .test:linux