Fully implemented reading the RTC century counter, and laid out initial...

Fully implemented reading the RTC century counter, and laid out initial infrastructure for ACPI information to be used across the kernel, in the x86_64 architecture.

- Implemented a global variable, ACPI_TABLE, behind a mutex, which contains the ACPI information pertinent to the rest of the kernel, currently solely containing a pointer to the FADT.
- Split device initialization into two categories - "core" devices, such as the PIC and local APIC, necessary for initializing the rest of the kernel, and "non-core" devices such as serial and RTC, which are to be initialized last.
- Checked for the presence of the century register, and consequentially read from, in the RTC code, now factored into the date calculation. The location of the register is pulled from the "century" field in the FADT.
- Modified page unmapping in the ACPI code, such that any tables to be stored globally (currently only the FADT) are not unmapped after reading, such that they can be stored in globally accessible pointers without causing page faults.