BPF: rename bpf_defaults to bpf_cc_defaults

With the addition of defaults to the bpf soong module (see aosp/3249485
), the name `bpf_defaults` will be used to define defaults for bpf
program compilation. While there are separate soong namespaces for
module types and module names, rename the existing bpf_defaults
cc_defaults to bpf_cc_defaults for clarity.

While we're at it, add -Wextra, and cleanup unnecessary cflags in
clauses which use bpf_cc_defaults.

Test: TH
Change-Id: Ib5f453ce33b13c698e8b91088bc59f8c6719c892
Signed-off-by: Neill Kapron <nkapron@google.com>