cmd/link: support PIE internal linking on darwin/amd64

This CL adds support of PIE internal linking on darwin/amd64.

This is also preparation for supporting internal linking on
darwin/arm64 (macOS), which requires PIE for everything.

Updates #38485.

Change-Id: I2ed58583dcc102f5e0521982491fc7ba6f2754ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/261642


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