[dev.link] cmd/link/internal/loader: change storage mechanism for sym alignment

Switch the storage mechanism for symbol alignment away from a map and
to a slice of uint8 values per symbol, where value K indicates
alignment 2^K. Intended to help speed up alignment get/set in dodata.

Change-Id: I26416e455c808f697dd0d7f6d2582247ee5c5b40
Reviewed-on: https://go-review.googlesource.com/c/go/+/231220


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>