perf test sigtrap: Generalize the BTF routine to reuse it in this test

perf test sigtrap: Generalize the BTF routine to reuse it in this test

Move the part that loads the BTF info to a "btf__available()" that will
lazy load the BTF info so that if we need it for some other test, which
we will in the following cset, we can reuse it.

At some point this will move from this specific 'perf test' entry to be
used in other parts of perf, do it when needed.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kate Carcia <kcarcia@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20231129154718.326330-2-acme@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>