The SUBZERO_USE_MICROSOFT_ABI macro definition was used to indicate that
we want to use the Microsoft x86-64 calling convention, instead of
the System V one which PNaCl assumes (even on Windows). Using the
standard _WIN64 macro instead makes us not require defining the custom
one as part of our build.

SUBZERO_USE_MICROSOFT_ABI was also being used to decide whether to emit
stack probes. For 32-bit Windows targets, use the _WIN32 macro instead.
Note that when _WIN64 is defined, _WIN32 is also defined, but to avoid
confusion we use _WIN64 in the X8664 backend.

Bug: b/179832693
Change-Id: Ic2e62d3dc26543876673c07b9ccc01e9d92762bf
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/55528
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>