X86: use the correct alloca symbol for Windows Itanium

Windows itanium targets the MSVCRT, and the stack probe symbol is provided by
MSVCRT.  This corrects the emission of stack probes on i686-windows-itanium.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222439 91177308-0d34-0410-b5e6-96231b3b80d8