Fixed inclusion of M_PI fow MinGW-w64

Patch by KOLANICH

llvm-svn: 346000
diff --git a/llvm/lib/Target/AMDGPU/SIISelLowering.cpp b/llvm/lib/Target/AMDGPU/SIISelLowering.cpp
index 55b1a87..88e07b9 100644
--- a/llvm/lib/Target/AMDGPU/SIISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/SIISelLowering.cpp
@@ -12,7 +12,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) || defined(__MINGW32__)
 // Provide M_PI.
 #define _USE_MATH_DEFINES
 #endif