x86: increment the stack pointer in lret inst

The 'lret' instruction reloads instruction pointer and code segment from the
stack and then pops them. But the popping part is missing from the current
implementation. This caused incorrect behavior in some code related to the
Fiasco OS. Microops are being added to rectify the behavior of the instruction.

Committed by: Nilay Vaish <nilay@cs.wisc.edu>

src/arch/x86/isa/insts/general_purpose/control_transfer/xreturn.py

@@ -69,6 +69,11 @@ def macroop RET_FAR {
     # Get the return CS
     ld t2, ss, [1, t0, rsp], ssz
 
+    # increment the stack pointer to pop the instruction pointer
+    # and the code segment from the stack.
+    addi rsp, rsp, dsz
+    addi rsp, rsp, dsz
+
     # Get the rpl
     andi t3, t2, 0x3