Refactoring in commit 77982de9 (CMakePackageConfigHelpers: only emit
arch check if needed, 2023-01-29, v3.26.0-rc1~16^2) did not correctly
preserve the check for empty `CMAKE_SIZEOF_VOID_P`.  Fix it.

The difference between the version files generated before and after is:

    -if(CMAKE_SIZEOF_VOID_P STREQUAL "" OR "8" STREQUAL "")
    +if("${CMAKE_SIZEOF_VOID_P}" STREQUAL "" OR "8" STREQUAL "")

This restores the check generated before the above-mentioned commit.

Fixes: #24608