arm64: Use __builtin_ms_va_list and __attribute__((ms_abi)) on arm64.

arm64: Use __builtin_ms_va_list and __attribute__((ms_abi)) on arm64.

Windows uses a different ABI for va_list on arm64 just like on x86_64.

On x86_64, the calling convention for windows functions is completely
different from the one on other platforms. On arm64, they're mostly the
same, with the only exception being variadic functions (where all float
arguments are passed in integer registers, since the va_list is a single
pointer).

Any functions using __builtin_ms_va_start need to be decorated with
__attribute__((ms_abi)).

Signed-off-by: Martin Storsjo <martin@martin.st>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>