cmd/link/internal/ld: bump NetBSD ABI version to 7.0

cmd/link/internal/ld: bump NetBSD ABI version to 7.0

According to https://golang.org/wiki/NetBSD, NetBSD 7.0 is supported as
of Go 1.3 (with Go 1.5 recommended). NetBSD 6.0 was last supported in Go
1.9.7. Thus, bump the minimal ABI version to NetBSD 7.0

Suggested by Benny Siegert in CL 212461.

Change-Id: I11dd14c6b835ac9fc156880b2551f71893c7f267
Reviewed-on: https://go-review.googlesource.com/c/go/+/220428
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>