Previously, different DWARF sections had relocations applied in very
different ways.  .debug_info was relocated, but only on x86-64 and 386
and using hard-coded relocation section names instead of relocation
links.  .debug_abbrev and .debug_str were never relocated (which is
excusable because they shouldn't need it).  .debug_types sections were
relocated on all architectures and found their relocation section
using a relocation link because section names could be ambiguous.

Simplify all of this so that every DWARF section that has a linked
relocation section gets those relocations applied.

This prepares this code to load .debug_line sections without the need
for yet more ad hoc relocation logic.

Change-Id: Ia00ac8e656b22f22bb31a5f6ef9b0f23cda64d19
Reviewed-on: https://go-review.googlesource.com/6780Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>