[build] Fix build failures with random versions of gcc

[build] Fix build failures with random versions of gcc

For unknown reasons, miscellaneous versions of gcc seem to struggle
with the static assertions used to ensure the correct layout of the
GCM structures.

Adjust the assertions to use offsetof() rather than direct pointer
comparison, on the basis that offsetof() must be a compile-time
constant value.

Signed-off-by: Michael Brown <mcb30@ipxe.org>