(scratch space is pre-allocated stack space since the IC can't
allocate new space itself, in order to have unwinding work)

Allocate a fixed amount (currently: 40 bytes) of extra stack space
in our runtime ics.  This involves changing the function prologue+
epilogue to do more rsp adjustment, and modifying the .eh_frame sections
we generate.

One tricky thing is that we currently use frame pointer elimination
in our runtime ics, but the rest of the scratch space logic had assumed
the scratch would be rbp-relative, which I had to convert to rsp-relative.