Add --print-supported-cpus flag for clang.

This patch allows clang users to print out a list of supported CPU models using
clang [--target=<target triple>] --print-supported-cpus

Then, users can select the CPU model to compile to using
clang --target=<triple> -mcpu=<model> a.c

It is a handy feature to help cross compilation.

llvm-svn: 363464
diff --git a/clang/docs/CommandGuide/clang.rst b/clang/docs/CommandGuide/clang.rst
index 84e0ddd..1ed11c7 100644
--- a/clang/docs/CommandGuide/clang.rst
+++ b/clang/docs/CommandGuide/clang.rst
@@ -324,6 +324,12 @@
   When building for iPhone OS, specify the minimum version supported by your
   application.
 
+.. 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 
+  specified, the system default target will be used.
+
 .. option:: -march=<cpu>
 
   Specify that Clang should generate code for a specific processor family