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

[Demetrius Kanios via llvm-commits]

QuantumSegfault wrote:

> How are the size and contents of this table determinted?

It is always a default (null) initialized, single-element table. It's always exactly the same.

Which is why weakly defined per-object is ideal.

And it's used in the backend in such a way that we can't rely on any particular library being linked in. i.e. This should work without issues

```ll
; bin/llc -filetype=obj -mattr=+reference-types -o - test.ll

target triple = "wasm32-unknown-unknown"

define void @call_callback(ptr addrspace(20) %callback) {
entry:
  call addrspace(20) void %callback()
  ret void
}
```

But `-filetype=obj` errors out: `LLVM ERROR: undefined table symbol cannot be weak`. But to get an idea (with the default `asm` file type)
```s
	.file	"test.ll"
	.tabletype	__funcref_call_table, funcref, 1
	.functype	call_callback (funcref) -> ()
	.section	.text.call_callback,"",@
	.globl	call_callback                   # -- Begin function call_callback
	.type	call_callback, at function
call_callback:                          # @call_callback
	.functype	call_callback (funcref) -> ()
# %bb.0:                                # %entry
	i32.const	0
	local.get	0
	table.set	__funcref_call_table
	i32.const	0
	call_indirect	 __funcref_call_table, () -> ()
	i32.const	0
	ref.null_func
	table.set	__funcref_call_table
                                        # fallthrough-return
	end_function
```

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