cmd/link/internal/ld: move more of mustLinkExternal into internal/platform

internal/platform.MustLinkExternal is used in various places to
determine whether external linking is required. It should always
match what the linker actually requires, but today does not match
because the linker imposes additional constraints.

Updates #31544.

Change-Id: I0cc6ad587e95c607329dea5d60d29a5fb2a9e722
Reviewed-on: https://go-review.googlesource.com/c/go/+/472515


Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>