[llvm-ar][test] Correct tests marked as expected fails

In diff D64802 I marked three tests as expected failures for darwin but
James Nagurne saw these fail on his downstream embedded ARM cross
compiler.
I believe XFAIL: system-darwin should be used instead of using XFAIL:
darwin due to the problem being related to the darwin host and not the
target.

Differential Revision: https://reviews.llvm.org/D65745

llvm-svn: 368564

llvm/test/tools/llvm-ar/extract.test

@@ -1,5 +1,5 @@
 ## Test extract operation.
-# XFAIL: darwin
+# XFAIL: system-darwin
 
 RUN: rm -rf %t && mkdir -p %t/extracted/
 

llvm/test/tools/llvm-ar/mri-utf8.test

@@ -1,5 +1,5 @@
 # Test non-ascii archive members
-# XFAIL: darwin
+# XFAIL: system-darwin
 
 RUN: rm -rf %t && mkdir -p %t/extracted
 

llvm/test/tools/llvm-ar/print.test

@@ -1,5 +1,5 @@
 ## Test Print output
-# XFAIL: darwin
+# XFAIL: system-darwin
 
 RUN: rm -rf %t && mkdir -p %t
 RUN: echo file1 > %t/1.txt