After restoring SCP->sc_mask, check for pending signals (newly unblocked);
if any, set SS->context to SCP, clear SS->intr_port, and send sig_post
to the signal thread to deliver the pending signals.
Don't write $1 value into the user stack. Instead, write it into the word
just past SCP->sc_pc; then point $1 at SCP->sc_pc and use `op_sigreturn'
pseudo-instruction to restore the PC and $1 from that.
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