Update symbol versioning for ffi_get_struct_offsets
Given that the original symbol versioning patch has shipped
with gcc-6, it would be kind to keep that as a valid base ABI
and make subsequent changes to version 7.1.
Use ELF symbol versioning
At the same time, we must bump the (major) ABI version. This needed to be
done anyway due to ABI breakage in the AArch64 port (see 12cf89ee and the
corresponding GCC PR70024).
