demangle-cxx.cpp requires a C++ compiler, but feature checks may fail
because of the absence of this. Add a CONFIG_CXX_DEMANGLE so that the
source isn't built if not supported. Copy libbfd and cplus demangle
variants to a weak symbol-elf.c version so they aren't dependent on
C++. These variants are only built with the build option
BUILD_NONDISTRO=1.

Committer note:

This also handles this build break when a C++ compiler isn't available:

  CXX     /tmp/build/perf/util/demangle-cxx.o
  /bin/sh: g++: command not found
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
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: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Qi Liu <liuqi115@huawei.com>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Link: https://lore.kernel.org/r/20230417192546.99923-1-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>