cmd/link: mark holes in functab with end PC-1

When we have multiple text sections, we need to mark holes between
the sections in the functab. A hole is marked with an entry with
the end PC of the previous section. As we now use offsets instead
of (relocated) PCs, the end offset of a section may be the same of
the start of the next one. Distinguish it by using the end address
-1.

For #48837.

Change-Id: I121aac53b32a869378632cf151cb1b6f98ad3089
Reviewed-on: https://go-review.googlesource.com/c/go/+/354636


Trust: Cherry Mui <cherryyz@google.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>