st/nine: specify WINAPI only for i386 and amd64

st/nine: specify WINAPI only for i386 and amd64

Currently mesa fails building with the x32 abi as ms_abi is not defined
in such a case.

The patch uses ms_abi only for amd64 targets and stdcall only for i386
targets to be sure that those are defined.

This patch additionally checks for __GNUC__ to guarantee that
__attribute__ is available.

CC: "11.1 11.2" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Christian Schmidbauer <ch.schmidbauer@gmail.com>
Acked-by: Axel Davy <axel.davy@ens.fr>