selftests/bpf: Use sys_pidfd_open() helper when possible

SYS_pidfd_open may be undefined for old glibc, so using sys_pidfd_open()
helper defined in task_local_storage_helpers.h instead to fix potential
build failure.

And according to commit 7615d9e1

 ("arch: wire-up pidfd_open()"), the
syscall number of pidfd_open is always 434 except for alpha architure,
so update the definition of __NR_pidfd_open accordingly.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221011071249.3471760-1-houtao@huaweicloud.com