Restore rsi on x64 when probing for CPU features.

BUG=v8:2238
TEST=

Review URL: https://chromiumcodereview.appspot.com/10801012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12126 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

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);