selftests/bpf: Add uprobe context registers changes test

selftests/bpf: Add uprobe context registers changes test

Adding test to check we can change common register values through
uprobe program.

It's x86_64 specific test.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20250916215301.664963-4-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>