site/elfutils/libunistring: Drop patching for iconv and set in site file

The standard macros from gettext for iconv include problematic tests which
we've been patching out adhoc. Stop doing this and set results in the site
files instead which is simpler, more maintainable and peforms better too
as an added bonus.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>