ELF: eliminate hard-coded section numbers

Eliminiate hard-coded section numbers, at least to the best of our
ability.  There is still a very odd piece of computation in
elf_build_reltab() which I can't really figure out...

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/doc/changes.src b/doc/changes.src
index 60a98e9..0e7899d 100644
--- a/doc/changes.src
+++ b/doc/changes.src
@@ -46,6 +46,9 @@
 
 \b The ELF backends no longer automatically generate a \c{.comment} section.
 
+\b Add additional "well-known" ELF sections with default attributes.  See
+   \k{elfsect}.
+
 
 \S{cl-2.05.01} Version 2.05.01