cmd/link: avoid name collision with DWARF .def suffix

cmd/link: avoid name collision with DWARF .def suffix

Adding a .def suffix for DWARF info collided with the DWARF info,
without the suffix, for a method named def. Change the suffix to ..def
instead.

Fixes #15926.

Change-Id: If1bf1bcb5dff1d7f7b79f78e3f7a3bbfcd2201bb
Reviewed-on: https://go-review.googlesource.com/23733
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>