[ELF] --gdb-index: error if constant pool size exceeds UINT32_MAX

If so, the last symbol's name_offset likely exceeds 0xffffffff and is not
supported by the format
(https://sourceware.org/gdb/onlinedocs/gdb/Index-Section-Format.html#Index-Section-Format).
I have seen an internal oversized executable with such a corrupted .gdb_index