[test]Mark thinlto icp test as unsupported on powerpc (#75979)

Test failed on ppc
(https://lab.llvm.org/buildbot/#/builders/231/builds/18902), and logs
shows missed import.

Cannot reproduce this with machines I could access so far.
https://gcc.gnu.org/wiki/CompileFarm seems to provide ppc64 machine.
Mark the thinlto icp test as unsupported for now.