xen/mm: Introduce {G,M}FN_INVALID_INITIALIZER

xen/mm: Introduce {G,M}FN_INVALID_INITIALIZER

The current implementation of {G,M}FN_INVALID cannot be used to
initialize global variable because the initializer element is not a
constant.

Due to a bug in GCC 4.9 and older ([1]), it is not easy to find a common
value to initialize a variable and directly passed as an argument.

Introduce 2 news define {G,M}FN_INVALID_INITIALIZER to be used for
initializing a variable.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64856

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Tim Deegan <tim@xen.org>