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

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@343013 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt