WriteBasicConfigVersionFile: Fix regression in ARCH_INDEPENDENT check

Refactoring in !8133 (merged) 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 (closed)
Backport: release