Kernel: optimize yielding

Doing a yield no longer raises a software interrupt. Instead it just
saves all the callee saved registers, ip, sp and return value. Because
yield is only called in the kernel, it can just restore registers and
jump to the target address. There is never a need to use iret :)

kernel/arch/i686/interrupts.S

@@ -83,28 +83,6 @@ irq_stub:
 	addl $8, %esp
 	iret
 
-.global asm_yield_handler
-asm_yield_handler:
-	# This can only be called from kernel, so no segment saving is needed
-	pushal
-	cld
-
-	leal 32(%esp), %edi # interrupt stack ptr
-	movl %esp,     %esi # interrupt registers ptr
-
-	movl %esp, %ebp
-	andl $-16, %esp
-
-	subl $8, %esp
-	pushl %esi
-	pushl %edi
-	call cpp_yield_handler
-
-	movl %ebp, %esp
-
-	popal
-	iret
-
 .global asm_ipi_handler
 asm_ipi_handler:
 	pushal