Instead, add a scratchFpMem field to ssafn,
so that it may be passed on to genssa.

Updates #15756

Change-Id: Icdeae290d3098d14d31659fa07a9863964bb76ed
Reviewed-on: https://go-review.googlesource.com/38728Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>