libxc: Expose xc_reserved_device_memory_map to ARM too

libxc: Expose xc_reserved_device_memory_map to ARM too

The commit 25652f2 "tools/libxl: detect and avoid conflicts with RDM"
introduced the usage of xc_reserved_device_memory_map in the libxl
generic code. But the function is only defined for x86 which breaks the
ARM build.

The hypercall called by this helper is implemented in the generic code
and doesn't contain any x86 specific code. Therefore, it's fine to
expose the helper to ARM.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
CC: Ian Jackson <ian.jackson@eu.citrix.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Tiejun Chen <tiejun.chen@intel.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>