selftests/bpf: Mark uprobe trigger functions with nocf_check attribute

Some distros seem to enable the -fcf-protection=branch by default,
which breaks our setup on first instruction of uprobe trigger
functions and place there endbr64 instruction.

Marking them with nocf_check attribute to skip that.

Ignoring unknown attribute warning in gcc for bench objects, because
nocf_check can be used only when -fcf-protection=branch is enabled,
otherwise we get a warning and break compilation.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240322134936.1075395-1-jolsa@kernel.org