cmd/link: preserve elf phdr flags when loading external objects

Preserve program header flags when passing them through loadelf.Load.
They shouldn't be coerced to 0 on non-ARM platforms which set them
such as ppc64le.

Change-Id: I022613356f910d812de2fc22eac949960eeb53b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/300950


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>