[dev.link] cmd/link/internal/loader: preprocess numeric constants earlier

[dev.link] cmd/link/internal/loader: preprocess numeric constants earlier

Change the timing for preprocessing of integer/floating point constant
symbols so that we populate them with content at an earlier stage.
This is needed to allow them can be picked up by the loader-API
version of dodata().

Change-Id: Icf09f4f4b318b4f77e11d4a0f0a9cbecd76a1d6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/229438
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>