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

[gbreynoo]

Summary

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.

[JamesNagurne]

Our team independently applied and validated this change for our ARM cross compiler running on darwin, and we shared the same failure modes as the darwin compiler. I feel that this is sufficient evidence that the bug is related to the host, not the target.

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

 ## Test extract operation.
-# XFAIL: darwin
+# XFAIL: system-darwin
 
 RUN: rm -rf %t && mkdir -p %t/extracted/
 
 # Extracting from an empty archive should not warn or error:
 RUN: llvm-ar cr %t/empty.a
 RUN: llvm-ar x %t/empty.a 2>&1 | count 0
 
 RUN: echo filea > %t/a.txt

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

 # Test non-ascii archive members
-# XFAIL: darwin
+# XFAIL: system-darwin
 
 RUN: rm -rf %t && mkdir -p %t/extracted
 
 RUN: echo "contents" > %t/£.txt
 
 RUN: echo "CREATE %t/mri.ar" > %t/script.mri
 RUN: echo "ADDMOD %t/£.txt" >> %t/script.mri
 RUN: echo "SAVE" >> %t/script.mri

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

 ## Test Print output
-# XFAIL: darwin
+# XFAIL: system-darwin
 
 RUN: rm -rf %t && mkdir -p %t
 RUN: echo file1 > %t/1.txt
 RUN: echo file2 > %t/2.txt
 RUN: echo file3 > %t/3.txt
 
 RUN: llvm-ar -rc %t/archive.a %t/1.txt %t/2.txt %t/3.txt