[dev.link] cmd/link: move findfunctab to a generated symbol

[dev.link] cmd/link: move findfunctab to a generated symbol

Basically removes all allocation from findfunctab:

Findfunctab_GC                172kB ± 0%       0kB ± 0%   ~     (p=1.000 n=1+1)

Change-Id: I246f7d2751317886b658f7ef672fb30b3c519668
Reviewed-on: https://go-review.googlesource.com/c/go/+/239281
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>