Create a new "AuxSymbol" struct into which 'cold' or 'infrequently
set' symbol fields are located. Move the Extname field from the
main Symbol struct to AuxSymbol.

Updates #26186

Change-Id: I9e795fb0cc48f978e2818475fa073ed9f2db202d
Reviewed-on: https://go-review.googlesource.com/125476Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>