[rts] Untag bq->bh prior to reading the info table

In `checkBlockingQueues` we must always untag the `bh` field of an `StgBlockingQueue`.
While at first glance it might seem a sensible assumption that `bh` will
always be a blackhole and therefore never be tagged, the GC could
shortcut the indirection and put a tagged pointer into the indirection.

This blew up on aarch64-darwin with a misaligned access. `bh` pointed
to an address that always ended in 0xa. On architectures that
are a little less strict about alignment, this would have read
a garbage info table pointer, which very, very unlikely would have been equal to
`stg_BLACKHOLE_info` and therefore things accidentally worked. However,
on AArch64, the read of the info table pointer resulted in a SIGBUS due
to misaligned read.

Fixes #20093.

(cherry picked from commit 1832676a)