Simplify task switches

No longer using the rsp from the entry to the kernel, but instead
switching rsp at task-switching time in assembly.

This currently breaks fork()

src/kernel/tasking.inc

@@ -0,0 +1,32 @@
+struc TCB
+.rsp:          resq 1
+.rsp0:		   resq 1
+.pml4:         resq 1
+endstruc
+
+struc CPU_DATA
+.rsp0:         resq 1
+.rsp3:         resq 1
+.tcb:          resq 1
+endstruc
+
+struc TSS
+.res0:         resd 1
+.rsp0:         resq 1
+.rsp1:         resq 1
+.rsp2:         resq 1
+.ist0:         resq 1
+.ist1:         resq 1
+.ist2:         resq 1
+.ist3:         resq 1
+.ist4:         resq 1
+.ist5:         resq 1
+.ist6:         resq 1
+.ist7:         resq 1
+.res1:         resq 1
+.res2:         resw 1
+.iomap:        resw 1
+endstruc
+
+
+; vim: ft=asm