Restore rsi on x64 when probing for CPU features.

src/x64/assembler-x64.cc

Index: src/x64/assembler-x64.cc
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index 89b971070ebdfeee26853233219fb6b0f0942545..77f92d844d5bd7798eb647082ba6545916709ea6 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -75,6 +75,7 @@ void CpuFeatures::Probe() {
   // Save old rsp, since we are going to modify the stack.
   __ push(rbp);
   __ pushfq();
+  __ push(rsi);
   __ push(rcx);
   __ push(rbx);
   __ movq(rbp, rsp);
@@ -128,6 +129,7 @@ void CpuFeatures::Probe() {
   __ movq(rsp, rbp);
   __ pop(rbx);
   __ pop(rcx);
+  __ pop(rsi);
   __ popfq();
   __ pop(rbp);
   __ ret(0);