cmd/link: unify text segment write

Currently we have two code paths of writing the text segment. They
are semantically the same:

- if we split text sections, we write all ".text" sections as
  text and the the rest as data.
- if we do not split text sections, we write the first section
  as text and the rest as data. The first section is named ".text"
  and is the only one in this case.

Unify the code.

Change-Id: Ic639eed625615be3c8a8d41f5b47e901552f587a
Reviewed-on: https://go-review.googlesource.com/c/go/+/316049


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>