MinGW: Define __stdcall&co when -fms-extensions is disabled

This is for compatibility with GCC.

Reviewers: asl

Differential Revision: http://reviews.llvm.org/D3444

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206791 91177308-0d34-0410-b5e6-96231b3b80d8