Begin the job of converting the linker's "gentext" phase over to use
loader APIs. This patch includes most architectures except for s390x
and PPC (these will be added in subsequent patches, since they require
a couple of loader changes first).

Change-Id: Ic7f55c207dcdbbba657330ef007a72ff7c837416
Reviewed-on: https://go-review.googlesource.com/c/go/+/227017


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