Correctly describe the stack during a GHCi CCALL instruction to the
RTS.  The previous hack, temporarily truncating the stack to the
topmost valid stack frame, didn't work because stack-squeezing tends
to move the stack around before the call.

The right thing to do is correctly describe the chunk of ccall args
with an info table, which is what this change does.  We use a RET_DYN
info table with the number of non-ptrs from the CCALL instruction.