internal/pkgbits: rename Reloc* to Section*

internal/pkgbits: rename Reloc* to Section*

This is a basic refactoring. This enumeration refers primarily to
the different sections of a UIR file, so this naming is a bit more
direct.

Change-Id: Ib70ab054e97effaabc035450d246ae4354da8075
Reviewed-on: https://go-review.googlesource.com/c/go/+/671935
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Mark Freeman <mark@golang.org>