ggml-cpu: BMI2 is only available on amd64

MSVC doesn't define __BMI2__ so it must be defined by by CMake. It was being set  whenever GGML_BMI2 is set, but that results in a failure (`error LNK2019: unresolved external symbol __pdep_u64`) when targeting x86 (aka 32 bit) Windows. The fix is to only set __BMI2__ when targeting amd64.

Author: candrews

ggml/src/ggml-cpu/CMakeLists.txt

@@ -308,7 +308,10 @@ function(ggml_add_cpu_backend_variant_impl tag_name)
             endif()
             if (GGML_BMI2)
                 # MSVC does not define macro __BMI2__
-                list(APPEND ARCH_DEFINITIONS __BMI2__ GGML_BMI2)
+                if (${CMAKE_SYSTEM_PROCESSOR} MATCHES "amd64")
+                    # BMI2 is only available on amd64
+                    list(APPEND ARCH_DEFINITIONS __BMI2__ GGML_BMI2)
+                endif()
             endif()
         else ()
             if (GGML_NATIVE)