cmd/link: support PIE on macOS/ARM64

On macOS/ARM64 everything must be PIE, and we already build PIE
in exe buildmode. Support PIE buildmode as well.

Updates #38485.

Change-Id: I10b68c2f6eb77714e31c26116c61a0e28bf9a358
Reviewed-on: https://go-review.googlesource.com/c/go/+/259442


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