init: fix build with older gcc

init: fix build with older gcc

__setup_str is used on arrays of char, so there aren't any relocatable
items. Hence __initconst has to be used here, not __initconstrel.

Whatever led to the revert of commit 59b151d2c0 (the original version
of a6066af5b1 "xen/init: Annotate all command line parameter
infrastructure as const" must have got addressed meanwhile - with the
patch here I can't see that old gcc (4.3ish) report a section type
conflict anymore.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>