Fix some build errors when building wamrc with LLVM-13, reported in #2311
Fix some build warnings when building wamrc with LLVM-16:
```
core/iwasm/compilation/aot_llvm_extra2.cpp:26:26: warning:
‘llvm::None’ is deprecated: Use std::nullopt instead. [-Wdeprecated-declarations]
26 | return llvm::None;
```
Fix a maybe-uninitialized compile warning:
```
core/iwasm/compilation/aot_llvm.c:413:9: warning:
‘update_top_block’ may be used uninitialized in this function [-Wmaybe-uninitialized]
413 | LLVMPositionBuilderAtEnd(b, update_top_block);
```
ea78b89965