tools build: Enable warnings through HOSTCFLAGS

The tools build system uses KBUILD_HOSTCFLAGS symbol for obvious purposes.

However this is not set for anything under tools/

As such, host tools apps built have no compiler warnings enabled.

Declare HOSTCFLAGS for perf tools build, and also use that symbol in
declaration of host_c_flags. HOSTCFLAGS comes from EXTRA_WARNINGS, which
is independent of target platform/arch warning flags.

Suggested-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Laura Abbott <labbott@kernel.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/1635525041-151876-1-git-send-email-john.garry@huawei.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>