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

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.

Differential Revision: https://reviews.llvm.org/D51883

llvm-svn: 343013