Kernel: Change ACPI & DMI definitions a bit

Structures declarations changed __attribute__((__packed__))
to [[gnu::packed]] in ACPI & DMI definitions.
Also, declarations of array of pointers in ACPI RSDT/XSDT are correct
now. In addition to that, now we have a declaration of the ACPI MADT
table & the table entries.

Author: supercomputer7