Add RETGUARD to clang for amd64. This security mechanism uses per-function
random cookies to protect access to function return instructions, with the
effect that the integrity of the return address is protected, and function
return instructions are harder to use in ROP gadgets.

On function entry the return address is combined with a per-function random
cookie and stored in the stack frame. The integrity of this value is verified
before function return, and if this check fails, the program aborts. In this way
RETGUARD is an improved stack protector, since the cookies are per-function. The
verification routine is constructed such that the binary space immediately
before each ret instruction is padded with int03 instructions, which makes these
return instructions difficult to use in ROP gadgets. In the kernel, this has the
effect of removing approximately 50% of total ROP gadgets, and 15% of unique
ROP gadgets compared to the 6.3 release kernel. Function epilogues are
essentially gadget free, leaving only the polymorphic gadgets that result from
jumping into the instruction stream partway through other instructions. Work to
remove these gadgets will continue through other mechanisms.

Index: include/llvm/InitializePasses.h
--- include/llvm/InitializePasses.h.orig
+++ include/llvm/InitializePasses.h
@@ -387,6 +387,7 @@ void initializeRemoveRedundantDebugValuesPass(PassRegi
 void initializeRenameIndependentSubregsPass(PassRegistry&);
 void initializeReplaceWithVeclibLegacyPass(PassRegistry &);
 void initializeResetMachineFunctionPass(PassRegistry&);
+void initializeReturnProtectorPass(PassRegistry&);
 void initializeReversePostOrderFunctionAttrsLegacyPassPass(PassRegistry&);
 void initializeRewriteStatepointsForGCLegacyPassPass(PassRegistry &);
 void initializeRewriteSymbolsLegacyPassPass(PassRegistry&);