perf build: Add Wextra for C++ compilation

Commit d58ac0bf8d1e ("perf build: Add clang and llvm compile and
linking support") added -Wall and -Wno-strict-aliasing for CXXFLAGS,
but not -Wextra. -Wno-strict-aliasing is no longer necessary, adding
-Wextra for CXXFLAGS requires adding -Wno-unused-parameter clang.cpp
and clang-test.cpp for LIBCLANGLLVM=1 to build.

Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: James Clark <james.clark@arm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Gaosheng Cui <cuigaosheng1@huawei.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.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: Rob Herring <robh@kernel.org>
Cc: Tom Rix <trix@redhat.com>
Cc: bpf@vger.kernel.org
Cc: llvm@lists.linux.dev
Link: https://lore.kernel.org/r/20230728064917.767761-4-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>