selftests/bpf: Reorg the uprobe_syscall test function

selftests/bpf: Reorg the uprobe_syscall test function

Adding __test_uprobe_syscall with non x86_64 stub to execute all the tests,
so we don't need to keep adding non x86_64 stub functions for new tests.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20250720112133.244369-13-jolsa@kernel.org