tools/resolve_btfids: Tidy HOST_OVERRIDES

Don't set EXTRA_CFLAGS to HOSTCFLAGS, ensure CROSS_COMPILE isn't
passed through.

Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20230202224253.40283-1-irogers@google.com