cmd/link/internal/ld: change pe64 from int to bool

cmd/link/internal/ld: change pe64 from int to bool

pe64 should be a bool, not a int. Probable a leftover from the
previous C implementation.

While here, us pe64 in more places.

Change-Id: Ie9871b39b64a7b9d317cb0700cb77a19ee23838d
Reviewed-on: https://go-review.googlesource.com/c/go/+/659115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>