apacheGH-44994: [C++][CMake] Use librt only for Linux (apache#44984)

### Rationale for this change

OpenBSD does not have `librt.so`.

### What changes are included in this PR?

Use `librt.so` only for Linux

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.

* GitHub Issue: apache#44994

Lead-authored-by: Brad Smith <brad@comstyle.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

cpp/src/arrow/CMakeLists.txt

@@ -166,8 +166,7 @@ if(WIN32)
   list(APPEND ARROW_SYSTEM_LINK_LIBS "ws2_32")
 endif()
 
-if(NOT WIN32 AND NOT APPLE)
-  # Pass -lrt on Linux only
+if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
   list(APPEND ARROW_SYSTEM_LINK_LIBS rt)
 endif()