xen: Introduce __initconst to store initial const data

xen: Introduce __initconst to store initial const data

It's possible to have 2 type (const and non-const) of data in the same
compilation unit. Using only __initdata will result to a compilation error:

    error: $variablename causes as section tupe conflict with $variablename2

because a section containing const variables is marked read only and so cannot
contain non-const variables.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Cambell <ian.campbell@citrix.com>
CC: Jan Beulich <JBeulich@suse.com>
CC: Keir Fraser <keir@xen.org>