Added an array for unary multiboot tags to be able to fetch them in constant-time

[NotYourFox]

Usage example:

struct multiboot_tag_mmap *multiboot_mmap =
	    (struct multiboot_tag_mmap *)get_multiboot_tag(MULTIBOOT_TAG_TYPE_MMAP);

The array is initialized in kernel_mmap_init(), with an exception to MULTIBOOT_TAG_TYPE_MODULE, since there may be more than one of them and a list-like structure should be made for them.