[ELF] Move elf::symtab into Ctx

Remove the global variable `symtab` and add a member variable
(`std::unique_ptr<SymbolTable>`) to `Ctx` instead.

This is one step toward eliminating global states.

Pull Request: https://github.com/llvm/llvm-project/pull/109612

lld/ELF/ScriptParser.cpp

@@ -1581,7 +1581,7 @@ Expr ScriptParser::readPrimary() {
     // script, it must happen before this DEFINED.
     auto order = ctx.scriptSymOrderCounter++;
     return [=, &ctx = this->ctx] {
-      Symbol *s = symtab.find(name);
+      Symbol *s = ctx.symtab->find(name);
       return s && s->isDefined() && ctx.scriptSymOrder.lookup(s) < order ? 1
                                                                          : 0;
     };