selftests/bpf: Clean up open-coded gettid syscall invocations

Availability of the gettid definition across glibc versions supported by
BPF selftests is not certain. Currently, all users in the tree open-code
syscall to gettid. Convert them to a common macro definition.

Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241104171959.2938862-3-memxor@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>