Check incompatible existing default symbol definition

After resolving a versioned reference, foo@VER1, to a default versioned
definition, foo@@VER1, from a shared object, we also merge it with
the existing regular default symbol definition, foo.  When foo is IFUNC
and foo@@VER1 aren't, we will merge 2 incompatible definitions.  This
patch avoids merging foo@@VER1 definition with foo definition if
one is IFUNC and the other isn't.