Build fixes: correct stale references from rename refactor
* Fix static function references broken by the _ prefix rename
pass across arch.c, apic.c, gdt.c, idt.c, irq.c, ps2.c, serial.c
* Fix mm/ references in heap.c, phys_window.c, physical.c
* Fix acpi.c and keyboard.c static call sites
* Fix init.c spawn function references
* Add missing includes to limits.h and signal.h