x86: Mark the RDTSC instruction as .serialize_before.

Change-Id: I20bf6a57ea4354aac9267845bb37b70b83d6fcde
Reviewed-on: https://gem5-review.googlesource.com/9042
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/arch/x86/isa/insts/system/msrs.py

@@ -60,6 +60,7 @@ def macroop WRMSR
 
 def macroop RDTSC
 {
+    .serialize_before
     rdtsc t1
     mov rax, rax, t1, dataSize=4
     srli t1, t1, 32, dataSize=8