[Bug 230846] dtrace FBT doesn't know about ifuncs

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=230846

            Bug ID: 230846
           Summary: dtrace FBT doesn't know about ifuncs
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: markj at FreeBSD.org

FBT creates entry/return probes for functions which have a corresponding ELF
symbol.  For e.g., copyout(), we'll get fbt::copyout{,_smap,_nosmap}:entry. 
Now users need to know which one to select.  In particular, fbt::copyout:entry
never fires now.

I think FBT should be made to treat STT_GNU_IFUNC symbols specially: when
enabling a probe on such a function, invoke the resolver to find the true
address, and put a probe there instead.

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