debug/elf: make check for empty symbol section consistent for 64-bit and 32-bit binaries

debug/elf: make check for empty symbol section consistent for 64-bit and 32-bit binaries

The check for whether a binary's symbols section is empty is
inconsistent across the 32-bit and 64-bit flows.

Change-Id: I1abc235320a53cf957cfb83c9e7bcad6e52bc529
GitHub-Last-Rev: f264915ca2964ad8f34ce1deee4f42c2f9dc21bf
GitHub-Pull-Request: golang/go#75334
Reviewed-on: https://go-review.googlesource.com/c/go/+/702195
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@google.com>