Add special case for unsupported test

That helps disable the test for all the targets

Author: hiraditya

llvm/docs/TestingGuide.rst

@@ -568,7 +568,8 @@ list of boolean expressions. The values in each expression may be:
 | ``UNSUPPORTED`` disables the test if any expression is true.
 | ``XFAIL`` expects the test to fail if any expression is true.
 
-As a special case, ``XFAIL: *`` is expected to fail everywhere.
+As a special case, ``XFAIL: *`` is expected to fail everywhere. Similarly,
+``UNSUPPORTED: target={{.*}}`` will disable the test everywhere.
 
 .. code-block:: llvm