[libunwind][AIX] Mark signal_frame.pass.cpp UNSUPPORTED on AIX

Summary:
This patch marks libunwind test case signal_frame.pass.cpp as UNSUPPORTED on AIX because the AIX assembler does not support CFI directives.

Reviewed by: danielkiss, MaskRay, ldionne, libunwind

Differential Revision: https://reviews.llvm.org/D113607