selftests/bpf: Fix open-coded gettid syscall in uprobe syscall tests

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.

The uprobe syscall tests got in from tip tree, using sys_gettid in there.

Fixes: 0e2fb011 ("selftests/bpf: Clean up open-coded gettid syscall invocations")
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>