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.

Credits to Andrew MacPherson.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193263 91177308-0d34-0410-b5e6-96231b3b80d8