Target/X86/MCTargetDesc/X86MCAsmInfo.cpp: Enable DwarfCFI (aka DW2) on Cygming.

Cygwin-1.7 supports dw2. Some recent mingw distros support one, too.
I have confirmed test-suite/SingleSource/Benchmarks/Shootout-C++/except.cpp can pass on Cygwin.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154247 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.html

@@ -384,6 +384,7 @@ Release Notes</a>.</h1>
   <li>Call instructions use the new register mask operands for faster compile
   times and better support for different calling conventions.  The old WINCALL
   instructions are no longer needed.</li>
+  <li>DW2 Exception Handling is enabled on Cygwin and MinGW.</li>
 </ul>
 
 </div>

lib/Target/X86/MCTargetDesc/X86MCAsmInfo.cpp

@@ -155,4 +155,7 @@ X86MCAsmInfoGNUCOFF::X86MCAsmInfoGNUCOFF(const Triple &Triple) {
   AssemblerDialect = AsmWriterFlavor;
 
   TextAlignFillValue = 0x90;
+
+  // Exceptions handling
+  ExceptionsType = ExceptionHandling::DwarfCFI;
 }