[Bug sanitizer/81645] New: Learn UBSAN to support -fsanitize=builtin

["marxin at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81645

            Bug ID: 81645
           Summary: Learn UBSAN to support -fsanitize=builtin
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marxin at gcc dot gnu.org
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org,
                    marxin at gcc dot gnu.org, mpolacek at gcc dot gnu.org
  Target Milestone: ---

I've just read LLVM weekly:

* The Undefined Behavior Sanitizer (UBSan) learned to detect undefined 
behaviour in calls to builtins. [r309459](http://reviews.llvm.org/rL309459).

And we can also support that. Currently they instrument CLZ and CZT builtins
family. But maybe there are also others that worth adding?