GH-47229: [C++][Arm] Force mimalloc to generate armv8.0 compatible binary

Mimalloc default generates LSE atomic instructions only work on armv8.1.
This causes illegal instruction on armv8.0 platforms like Raspberry4.
This PR sets mimalloc build flag -DMI_NO_OPT_ARCH=ON to disable LSE
instruction.
Please note even with flag set, compiler and libc will replace the
atmoic call with an ifunc that matches hardware best at runtime. That
means LSE is used only if the running platform supports it.

Author: cyb70289

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -2294,7 +2294,8 @@ if(ARROW_MIMALLOC)
       -DMI_LOCAL_DYNAMIC_TLS=ON
       -DMI_BUILD_OBJECT=OFF
       -DMI_BUILD_SHARED=OFF
-      -DMI_BUILD_TESTS=OFF)
+      -DMI_BUILD_TESTS=OFF
+      -DMI_NO_OPT_ARCH=ON)
 
   externalproject_add(mimalloc_ep
                       ${EP_COMMON_OPTIONS}