[lld][ELF] Use size_t for getNumSymbols return type (#230760)
Fix 32-bit overflow in `SymbolTableBaseSection::getSize` by changing
`getNumSymbols` to return size_t.
The overflow happens in a real large binary with 258 million symbols.
With 24-byte ELF64 symbol entries, the symbol table size exceeds 4GiB.