Add comment pointing to the binutils bugzilla entry

This is a follow-up to r203635 as suggested by Rafael.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203670 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/ARM/MCTargetDesc/ARMMCAsmInfo.cpp

@@ -56,5 +56,6 @@ ARMELFMCAsmInfo::ARMELFMCAsmInfo() {
   UseIntegratedAssembler = true;
 
   // gas doesn't handle VFP register names in cfi directives.
+  // See https://sourceware.org/bugzilla/show_bug.cgi?id=16694
   DwarfRegNumForCFI = true;
 }