[efi] Use standard va_args macros instead of VA_START() etc

[efi] Use standard va_args macros instead of VA_START() etc

The EDK2 header macros VA_START(), VA_ARG() etc produce build errors
on some CPU architectures (notably on 32-bit RISC-V, which is not yet
supported by EDK2).

Fix by using the standard variable argument list macros.

Signed-off-by: Michael Brown <mcb30@ipxe.org>