[llvm] [WebAssembly] Define `__funcref_call_table` in generated asm and objects (PR #180900)

[Derek Schuff via llvm-commits]

dschuff wrote:

I think we should maybe be able to treat this like the regular function table symbol (`__indirect_function_table`) that's used (when the CallIndirectOverlong feature is enabled). In that case that symbol is used in the same way; each call_indirect has a relocation against that symbol. It looks like `WebAssembly::getOrCreateFuncrefCallTableSymbol` is just defining the table as weak on creation rather than ever leaving it undefined.

https://github.com/llvm/llvm-project/pull/180900