[WebAssembly] Refactor linker-generated symbols. NFC.

Group all synthetic symbols in the in single struct to match
the ELF linker.

This change is part of a larger change to add more linker
symbols such as `_end` and `_edata`.

Differential Revision: https://reviews.llvm.org/D42866

llvm-svn: 324157
diff --git a/lld/wasm/MarkLive.cpp b/lld/wasm/MarkLive.cpp
index 9bd2a83..7a0a9e9 100644
--- a/lld/wasm/MarkLive.cpp
+++ b/lld/wasm/MarkLive.cpp
@@ -52,7 +52,7 @@
   // Add GC root symbols.
   if (!Config->Entry.empty())
     Enqueue(Symtab->find(Config->Entry));
-  Enqueue(Config->CtorSymbol);
+  Enqueue(WasmSym::CallCtors);
 
   // By default we export all non-hidden, so they are gc roots too
   for (Symbol *Sym : Symtab->getSymbols())