cmd/link: set correct flags in .dynamic for PIE buildmode

For internal linking, when generating a PIE binary, set the proper
.dynamic section flags to mark the binary as position-independent.

Fixes #46747.

Change-Id: I2b899148c6d06f92c9d12257a9761278b4236dfc
Reviewed-on: https://go-review.googlesource.com/c/go/+/328089


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Trust: Than McIntosh <thanm@google.com>