[dev.link] cmd/internal/dwarf: revise Abbrevs() signature

[dev.link] cmd/internal/dwarf: revise Abbrevs() signature

The function Abbrevs() was returning an array of structures by value,
which is not very efficient (this was showing up in a kubernetes
kubelet linker profile). Switch the function to return a slice
instead.

Improves linker DwarfGenerateDebugSyms running time when
linking the compiler in compilebench:

DwarfGenerateDebugSyms   29.2ms ±144%  23.9ms ±125%  -17.89%  (p=0.000 n=99+99)

Change-Id: I1132816563f208c63eb82a7932d9f2bcb2455324
Reviewed-on: https://go-review.googlesource.com/c/go/+/231558
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>