internal/platform: add a function to report whether default builds are PIE

internal/platform: add a function to report whether default builds are PIE

This consolidates a condition that was previously repeated (in
different approximations) in several different places in the code.

For #58807.

Change-Id: Idd308759f6262b1f5c61f79022965612319edf94
Reviewed-on: https://go-review.googlesource.com/c/go/+/475457
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>