Preserve VASigCookieReg across PInvokeStubWorker call (#17521)
Preserve VASigCookieReg across PInvokeStubWorker call (#17521)

The call to PInvokeStubWorker can do all kinds of stuff to the
VASigCookieReg in the GenericPInvokeCalli case, since x15 is just a
temporary register. Let's save it in a callee-saved register so that
when we come back after stub generation, we still have the correct value
for the VASigCookie.
cshung/coreclr,cshung/coreclr,krk/coreclr,wtgodbe/coreclr,krk/coreclr,krk/coreclr,mmitche/coreclr,poizan42/coreclr,poizan42/coreclr,wtgodbe/coreclr,wtgodbe/coreclr,mmitche/coreclr,mmitche/coreclr,krk/coreclr,mmitche/coreclr,krk/coreclr,cshung/coreclr,krk/coreclr,wtgodbe/coreclr,cshung/coreclr,wtgodbe/coreclr,wtgodbe/coreclr,poizan42/coreclr,poizan42/coreclr,cshung/coreclr,cshung/coreclr,mmitche/coreclr,mmitche/coreclr,poizan42/coreclr,poizan42/coreclr