MipsAsmParser/O32: Don't add redundant $ to $-prefixed symbol in the la macro (#80644)

When parsing the `la` macro, we add a duplicate `$` prefix in
`getOrCreateSymbol`,
leading to `error: Undefined temporary symbol $$yy` for code like:

```
xx:
	la	$2,$yy
$yy:
	nop
```

Remove the duplicate prefix.

In addition, recognize `.L`-prefixed symbols as local for O32.

See: #65020.

---------

Co-authored-by: Fangrui Song <i@maskray.me>