use a GCC equivalent for __stdcall where applicable

use a GCC equivalent for __stdcall where applicable

GCC on 32-bit architectures has __attribute__((stdcall)) as
an equivalent to Windows __stdcall, so use it conditionally
instead of leaving it blank for all non-Windows platforms.

This may benefit all platforms where GCC or compatible
compilers are used, including (but not limited to) Linux.