cmd/compile: add debug log prior to export data lookup

cmd/compile: add debug log prior to export data lookup

If there is a crash in LookupFunc (which has occurred a few times now),
this ensures that we log the offending symbol before crashing.

For #67016.

Change-Id: I0119597de2be3d1b97c41a9361273d1feb90ec11
Reviewed-on: https://go-review.googlesource.com/c/go/+/581437
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>