x86: fix SS selector in SYSRET

According to my reading of the Intel documentation, the SYSRET instruction
is supposed to force the RPL bits of the %ss register to 3 when returning
to user mode. The actual sequence is:

SS.Selector <-- (IA32_STAR[63:48]+8) OR 3; (* RPL forced to 3 *)

However, the code in helper_sysret() leaves them at 0 (in other words, the "OR
3" part of the above sequence is missing). It does set the privilege level
bits of %cs correctly though.

This has caused me trouble with some of my VxWorks development: code that runs
okay on real hardware will crash on QEMU, unless I apply the patch below.
Signed-off-by: NBill Paul <wpaul@windriver.com>
Message-Id: <201503091548.01462.wpaul@windriver.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

target-i386/seg_helper.c

@@ -1043,7 +1043,7 @@ void helper_sysret(CPUX86State *env, int dflag)
                                    DESC_CS_MASK | DESC_R_MASK | DESC_A_MASK);
             env->eip = (uint32_t)env->regs[R_ECX];
         }
-        cpu_x86_load_seg_cache(env, R_SS, selector + 8,
+        cpu_x86_load_seg_cache(env, R_SS, (selector + 8) | 3,
                                0, 0xffffffff,
                                DESC_G_MASK | DESC_B_MASK | DESC_P_MASK |
                                DESC_S_MASK | (3 << DESC_DPL_SHIFT) |
@@ -1056,7 +1056,7 @@ void helper_sysret(CPUX86State *env, int dflag)
                                DESC_S_MASK | (3 << DESC_DPL_SHIFT) |
                                DESC_CS_MASK | DESC_R_MASK | DESC_A_MASK);
         env->eip = (uint32_t)env->regs[R_ECX];
-        cpu_x86_load_seg_cache(env, R_SS, selector + 8,
+        cpu_x86_load_seg_cache(env, R_SS, (selector + 8) | 3,
                                0, 0xffffffff,
                                DESC_G_MASK | DESC_B_MASK | DESC_P_MASK |
                                DESC_S_MASK | (3 << DESC_DPL_SHIFT) |