cmd/link: revert DWARF version to 2 for .debug_lines

cmd/link: revert DWARF version to 2 for .debug_lines

On OSX 10.12 and earlier, paired with XCode 9.0,
specifying DWARF version 3 causes dsymutil to misbehave.
Version 2 appears to be good enough to allow processing
of the prologue_end opcode on (at least one version of)
Linux and OSX 10.13.

Fixes #25451.

Change-Id: Ic760e34248393a5386be96351c8e492da1d3413b
Reviewed-on: https://go-review.googlesource.com/114015
Reviewed-by: Alessandro Arzilli <alessandro.arzilli@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>