Add host-triplet aliasing feature to mkheader.

* src/Makefile.am (lock_obj_pub): Rename i586-pc-linux-gnu to
i686-pc-linux-gnu.  Remove i486-pc-linux-gnu.
* src/mkheader.c (canon_host_triplet): New.
(main): Use it.

config.sub does not map i{4,5,6}86-pc-linux-gnu to one common triplet.
However, they all use the same ABI and thus we do not need several
versions of the syscfg files.

• Signed-off-by: Werner Koch <wk@gnupg.org>