tools: Bump minimum LLVM C++ std to GNU++14

tools: Bump minimum LLVM C++ std to GNU++14

LLVM 9 (current release is LLVM 13) moved the minimum C++ version to
GNU++14. Bump the version numbers in the feature test and perf build.

Reviewed-by: Fangrui Song <maskray@google.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
Cc: llvm@lists.linux.dev
Link: https://lore.kernel.org/r/20211012021321.291635-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>