cmd/link: do not mark holes in functab

With multiple text sections, there may be holes (non-Go code) in
the PC range of Go code and covered by the functab. Previously, we
use a linear search with actual PCs to find the functab entry. We
need to use special entries to mark holes, so a PC in the hole can
be distinguished from the previous function.

Now, with the previous CL we find if the PC is in between of the
sections upfront in textOff. There is no need to mark holes in the
functab.

Change-Id: I22ff27279422bfc855c2ca35ba0fdfb63234c113
Reviewed-on: https://go-review.googlesource.com/c/go/+/354874


Trust: Cherry Mui <cherryyz@google.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>