[flang-rt] replace the triple dir to 'aix' for flang-rt to be consistent with clang on AIX. (#130875)

This change is to put `libflang_rt.runtime.a` into
`build/lib/clang/21/lib/aix/libflang_rt.runtime.a`, which is consistent
with clang on AIX.

Author: DanielCChen

flang-rt/cmake/modules/GetToolchainDirs.cmake

@@ -34,8 +34,9 @@
 function (get_toolchain_library_subdir outvar)
   set(outval "lib")
 
-  if (APPLE)
+  if (APPLE OR (UNIX AND CMAKE_SYSTEM_NAME MATCHES "AIX"))
     # Required to be "darwin" for MachO toolchain.
+    # AIX uses lib/${os_dir} as if LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF
     get_toolchain_os_dirname(os_dirname)
     set(outval "${outval}/${os_dirname}")
   else ()