[cfi] Mark tests as xfailing on Darwin

This allows them to be run on other platforms, undoing damage from
r286902.

llvm-svn: 286904

compiler-rt/test/cfi/icall/external-call.c

@@ -4,7 +4,7 @@
 
 // This test uses jump tables containing PC-relative references to external
 // symbols, which the Mach-O object writer does not currently support.
-// REQUIRES: rdar://problem/29255437
+// XFAIL: darwin
 
 #include <stdlib.h>
 #include <stdio.h>

compiler-rt/test/cfi/icall/weak.c

@@ -1,6 +1,6 @@
 // Test that weak symbols stay weak.
 // RUN: %clang_cfi -lm -o %t1 %s && %t1
-// REQUIRES: rdar://problem/29255437
+// XFAIL: darwin
 
 __attribute__((weak)) void does_not_exist(void);