wasm: use MVar as JSFFI import blocking mechanism

Previously, when blocking on a JSFFI import, we push a custom
stg_jsffi_block stack frame and arrange the `promise.then` callback to
write to that stack frame. It turns out we can simply use the good old
MVar to implement the blocking logic, with a few benefits:

- Less maintenance burden. We can drop the stg_jsffi_block related Cmm
  code without loss of functionality.
- It interacts better with existing async exception mechanism. throwTo
  would properly block the caller if the target thread is masking
  async exceptions.

(cherry picked from commit 9cd9f347)