[Sanitizers] [MinGW] Check for __i386__ in addition to _M_IX86 for i386 specific details

[mstorsjo]

Summary

The MinGW headers do define _M_IX86 (contrary to clang-cl and MSVC where it is a compiler predefined macro), but the headers that define it aren't included here.

Also check these defines for setting the symbol prefix, instead of inconsistently using _WIN64 for that.

compiler-rt/trunk/lib/sanitizer_common/sanitizer_win_defs.h

 //===----------------------------------------------------------------------===//
 #ifndef SANITIZER_WIN_DEFS_H
 #define SANITIZER_WIN_DEFS_H
 
 #include "sanitizer_platform.h"
 #if SANITIZER_WINDOWS
 
 #ifndef WINAPI
-#ifdef _M_IX86
+#if defined(_M_IX86) || defined(__i386__)
 #define WINAPI __stdcall
 #else
 #define WINAPI
 #endif
 #endif
 
-#if defined(_WIN64)
-#define WIN_SYM_PREFIX
-#else
+#if defined(_M_IX86) || defined(__i386__)
 #define WIN_SYM_PREFIX "_"
+#else
+#define WIN_SYM_PREFIX
 #endif
 
 // Intermediate macro to ensure the parameter is expanded before stringified.
 #define STRINGIFY_(A) #A
 #define STRINGIFY(A) STRINGIFY_(A)
 
 // ----------------- A workaround for the absence of weak symbols --------------
 // We don't have a direct equivalent of weak symbols when using MSVC, but we can