efi_loader: Introduce ms abi vararg helpers

Varargs differ between sysv and ms abi. On x86_64 we have to follow the ms
abi though, so we also need to make sure we use x86_64 varargs helpers.

This patch introduces generic efi vararg helpers that adhere to the
respective EFI ABI. That way we can deal with them properly from efi
loader code and properly interpret variable arguments.

This fixes the InstallMultipleProtocolInterfaces tests in the efi selftests
on x86_64 for me.

Signed-off-by: Alexander Graf <agraf@suse.de>