[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

libunwind/test/signal_frame.pass.cpp

@@ -17,6 +17,10 @@
 
 // UNSUPPORTED: libunwind-arm-ehabi
 
+// The AIX assembler does not support CFI directives, which
+// are necessary to run this test.
+// UNSUPPORTED: target=powerpc{{(64)?}}-ibm-aix
+
 #include <assert.h>
 #include <stdlib.h>
 #include <libunwind.h>