selftests/bpf: More open-coded gettid syscall cleanup

Commit 0e2fb011 ("selftests/bpf: Clean up open-coded gettid syscall
invocations") addressed the issue that older libc may not have a gettid()
function call wrapper for the associated syscall.

A few more instances have crept into tests, use sys_gettid() instead, and
poison raw gettid() usage to avoid future issues.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20250911163056.543071-1-alan.maguire@oracle.com