selftests/bpf: rename and clean up userspace-triggered benchmarks

Rename uprobe-base to more precise usermode-count (it will match other
baseline-like benchmarks, kernel-count and syscall-count). Also use
BENCH_TRIG_USERMODE() macro to define all usermode-based triggering
benchmarks, which include usermode-count and uprobe/uretprobe benchmarks.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240326162151.3981687-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>