Fixed the --print-supported-cpus test

Add constraints for the test that require specific backend targets
to be registered.

Remove trailing whitespace in the doc.

Differential Revision: https://reviews.llvm.org/D63105

llvm-svn: 363475
diff --git a/clang/docs/CommandGuide/clang.rst b/clang/docs/CommandGuide/clang.rst
index 1ed11c7..f511022 100644
--- a/clang/docs/CommandGuide/clang.rst
+++ b/clang/docs/CommandGuide/clang.rst
@@ -326,8 +326,8 @@
 
 .. option:: --print-supported-cpus
 
-  Print out a list of supported processors for the given target (specified 
-  through --target=<architecture> or -arch <architecture>). If no target is 
+  Print out a list of supported processors for the given target (specified
+  through --target=<architecture> or -arch <architecture>). If no target is
   specified, the system default target will be used.
 
 .. option:: -march=<cpu>