Add patch to replace open with openat with AT_FDCWD

Add patch to replace open with openat with AT_FDCWD

This patch replaces calls to the open systemcall with the openat systemcall
with the AT_FDCWD argument if SYS_open is not defined, similar to the
original musl code.

This is necessary as the open systemcall is not available on aarch64.

Signed-off-by: Robert Kuban <robert.kuban@opensynergy.com>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Razvan Virtan <virtanrazvan@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #6