selftests/bpf: skip test_perf_branches_hw() on unsupported platforms

selftests/bpf: skip test_perf_branches_hw() on unsupported platforms

Gracefully skip the test_perf_branches_hw subtest on platforms that
do not support LBR or require specialized perf event attributes
to enable branch sampling.

For example, AMD's Milan (Zen 3) supports BRS rather than traditional
LBR. This requires specific configurations (attr.type = PERF_TYPE_RAW,
attr.config = RETIRED_TAKEN_BRANCH_INSTRUCTIONS) that differ from the
generic setup used within this test. Notably, it also probably doesn't
hold much value to special case perf event configurations for selected
micro architectures.

Fixes: 67306f84ca78c ("selftests/bpf: Add bpf_read_branch_records() selftest")
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20251120142059.2836181-1-mattbobrowski@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>