Rewrote physical memory allocator

The physical memory allocator now uses a buddy allocator instead of a stack.

Also moved some of the platform-independent context code to kernel.c.

src/x86/load_context.S

@@ -7,8 +7,8 @@
 .type load_context, @function
 load_context:
     mov 4(%esp), %eax
-    push 0x1C(%eax)
     push 0x20(%eax)
+    push 0x1C(%eax)
     push 0x2C(%eax)
     push 0x24(%eax)
     push 0x28(%eax)
@@ -19,10 +19,11 @@ load_context:
     mov 0x10(%eax), %edi
     mov 0x14(%eax), %esi
     mov 0x18(%eax), %ebp
-    mov 0x1C(%eax), %ax
+    mov 0x20(%eax), %ax
     mov %ax, %ds
     mov %ax, %es
     mov %ax, %fs
     mov %ax, %gs
     pop %eax
-    iret
+    iret
+.size load_context, . - load_context