[dev.link] cmd/link: record external symbol alignment

Add a mechanism for recording symbol alignment for external symbols
under the new loader scheme. Alignments is stored in a side table,
since most symbols don't wind up needing an alignment other than zero.

Change-Id: I97092481412c15eac9b9f4c29b5c273f53759562
Reviewed-on: https://go-review.googlesource.com/c/go/+/210177


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>