If supported by target, internal symbols of libc.so should be accessed
without PLT nor GOT.  X86 PLT relocation is checked on libc_pic.a:

[hjl@gnu-efi-2 build-x86_64-linux]$ readelf -rW libc_pic.a | grep " __" | grep PLT32  | awk '{ print $5 }' | sort | uniq

It assumes that only symbols with "__" prefix are internal symbols.  A
machine-independent script should not be checking names
of relocations.  Instead it needs to look at properties of (global/weak) 
symbols in objects that go into shared libraries.  Symbols (defined or 
undefined) that are not hidden, but are not exported from those shared 
libraries, are suspect, but I'm not sure how reliably GCC will mark 
undefined symbols hidden even given appropriate declarations (and there 
may also be issues with undefined symbols properly called from .S sources 
but not marked hidden there).  So likely lists of exceptions to any such 
tests would be needed (including architecture-specific exceptions for e.g. 
libgcc symbols).