doc/go1.15: clarify external linking can still be used for building PIE

In Go 1.15 we switched the default linking mode for PIE on
Linux/AMD64 and Linux/ARM64 to internal linking. Clarify that
the previous behavior (external linking) can still be used with
a flag.

Fixes #40719.

Change-Id: Ib7042622bc91e1b1aa31f520990d03b5eb6c56bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/248199


Reviewed-by: Ian Lance Taylor <iant@golang.org>