Add support for the "merge" attribute in ELF, along with the
associated "strings" and size specifier attributes.
Fix a few places where we used "int", but a larger type really ought
to have been used.
Be a bit more lax about respecifying attributes. For example, align=
can be respecified; the highest resulting value is used.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>