Cannot link shared libaries which import `thread.new-indirect`

[alexcrichton]

Given this foo.wit:

package a:b;

world hi {
  export hi: func();
}

and this foo.wat

(module
  (import "$root" "[thread-new-indirect-v0]" (func (param i32 i32) (result i32)))

  (func (export "hi"))
)

this yields:

$ wasm-tools component link <(wasm-tools component embed foo.wit foo.wat)
error: failed to encode a component from modules

Caused by:
    0: table __indirect_function_table must be an exported funcref table for thread.new-indirect

cc @dicej, would you be up for taking this?