(this is a corrected patch)

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 OS (both Windows target and MingW target) 
but not Mach-O object format: Looks like macho environment was used to 
build some EFI code.
 
Credits to Andrew MacPherson.

llvm-svn: 193289