ARM: Add -m[no-]crc to dis/enable CRC subtargetfeature from clang

Allow users to disable or enable CRC subtarget feature.

Differential Revision: http://llvm-reviews.chandlerc.com/D2037

llvm-svn: 193600
diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index 88b3b0f..4dc8bbf 100644
--- a/clang/docs/UsersManual.rst
+++ b/clang/docs/UsersManual.rst
@@ -1057,6 +1057,15 @@
    hardware division instructions. This only applies to the ARM
    architecture.
 
+.. option:: -m[no-]crc
+
+   Enable or disable CRC instructions.
+
+   This option is used to indicate whether CRC instructions are to
+   be generated. This only applies to the ARM architecture.
+
+   CRC instructions are enabled by default on ARMv8.
+
 
 Controlling Size of Debug Information
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