Revert "Calling _chkstk is required on ELF as well as COFF on Windows. Without...

Revert "Calling _chkstk is required on ELF as well as COFF on Windows. Without _chkstk functions requiring large stack crash in initialization code. Previous code tested for COFF format but not Mach-O and this patch modifies the code to test for Windows."

This reverts commit r193263.

It is causing CodeGen/X86/mingw-alloca.ll to fail.

llvm-svn: 193275