[dev.link] cmd/link: set direct fn address in dwarf gen where possible

[dev.link] cmd/link: set direct fn address in dwarf gen where possible

If we are internal linking a static executable, and address assignment
has happened, then when emitting some parts of DWARF we can just emit
a function address directly instead of generating a relocation. For
external linking or other build modes, we are generating a relocatable
binary so we still need to emit relocations.

This CL inspired by Cherry's similar CL for pclntab at
https://go-review.googlesource.com/c/go/+/228478.

Change-Id: Ib03fbe2dd72d0ba746bf46015e0f2d6c3f3d53ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/228537
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>