cmd/link/internal/loadelf: better error message for ignored symbols

Currently it's quite hard to debug these error messages about ignored symbols
because there are only some numbers and no symbol name. Add symbol name. Before:
135029: sym#952: ignoring symbol in section 11 (type 0)
After:
135029: sym#952 (_ZN11__sanitizer9SpinMutexC5Ev): ignoring symbol in section 11 (type 0)

Change-Id: I7fec50b5798068c74827376613be529803838c5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/363034


Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Dmitry Vyukov <dvyukov@google.com>