kernel/amd64: Improve GDT implementation

* Switched to using macros for the GDT entry bits, to improve clarity
  and make the code easier to modify. Also got rid of some junk values
  in a few of the entries.
* Shrunk the GDT data array from 256 entries to 7 (GDT_ENTRY_COUNT)
  since the GDT will only ever use 7 of the entries for now.
* Aligned the GDT using __cacheline_aligned, because the GDT entries
  will be read every time a segment selector is loaded.
* Modified code dealing with the GDTR to just use `g_gdtr`, since the
  same GDT/GDTR is always used.
* In `gdt_load()`, changed `eax` -> `ax`, since segment registers are
  only 16 bits. Also replaced the `gdtr` to just use `g_gdtr`.
  Additionally, used the inline assembly formatting to supply segment
  selectors with KERNEL_CS/DS macros.

Signed-off-by: Quinn Stephens <quinn@osmora.org>
Signed-off-by: Ian Moffett <ian@osmora.org>

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