perf build: Make BUILD_BPF_SKEL default, rename to NO_BPF_SKEL

perf build: Make BUILD_BPF_SKEL default, rename to NO_BPF_SKEL

BPF skeleton support is now key to a number of perf features. Rather
than making it so that BPF support must be enabled for the build, make
this the default and error if the build lacks a clang and libbpf that
are sufficient. To avoid the error and build without BPF skeletons the
NO_BPF_SKEL=1 flag can be used. Add a build-options flag to 'perf
version' to enable detection of the BPF skeleton support and use this
in the offcpu shell test.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Martin Liška <mliska@suse.cz>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Pavithra Gurushankar <gpavithrasha@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Monnet <quentin@isovalent.com>
Cc: Roberto Sassu <roberto.sassu@huawei.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Tom Rix <trix@redhat.com>
Cc: Yang Jihong <yangjihong1@huawei.com>
Cc: llvm@lists.linux.dev
Link: https://lore.kernel.org/r/20230311065753.3012826-2-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>