Enable StackMap Serialization for COFF

Summary

This change turns on the emission of 
__LLVM_Stackmaps section when generating COFF binaries.

Test Plan

Added a scenario to the test case: 
test\CodeGen\X86\statepoint-stackmap-format.ll.

Code Review:

http://reviews.llvm.org/D10680

llvm-svn: 240613