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

Author: DanielCChen

flang-rt/cmake/modules/GetToolchainDirs.cmake

@@ -118,6 +118,9 @@ function (get_toolchain_arch_dirname outvar)
     set(target "amdgcn-amd-amdhsa")
   elseif("${arch}" MATCHES "^nvptx")
     set(target "nvptx64-nvidia-cuda")
+  elseif(UNIX AND CMAKE_SYSTEM_NAME MATCHES "AIX")
+    # Put at lib/aix to be consistent with clang on AIX.
+    string(TOLOWER "${CMAKE_SYSTEM_NAME}" target)
   else()
     set(target "${arch}${triple_suffix}")
   endif()