[libc++][test] Mark ranges.transform.pass.cpp UNSUPPORTED for AIX

The `ranges.transform.pass.cpp` often times out on CI for AIX (32-bit and 64-bit)
only. Mark the test as `UNSUPPORTED` for `AIX` for now. It should be looked into in
the future.

Differential Revision: https://reviews.llvm.org/D127051

Author: JoeLoser

libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/ranges.transform.pass.cpp

@@ -8,6 +8,8 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-has-no-incomplete-ranges
+// On AIX the test takes a long time, leading to frequent CI timeouts.
+// UNSUPPORTED: LIBCXX-AIX-FIXME
 
 // <algorithm>