PNaCl does not use the Microsoft x86-64 calling convention on Windows,
so the System V convention is the new default for Subzero. We need to
set a compile definition to explicitly request the use of the
Microsoft convention.

Change-Id: Ic70f5015d4b5753aa08a1247b6ac73eb9a9d278c
Reviewed-on: https://swiftshader-review.googlesource.com/8378Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>