libbpf: Initialize CFLAGS before including Makefile.include

libbpf: Initialize CFLAGS before including Makefile.include

tools/scripts/Makefile.include may expand EXTRA_CFLAGS in a future
change.  This could alter the initialization of CFLAGS, as the default
options "-g -O2" would never be set once EXTRA_CFLAGS is expanded.

Prepare for this by moving the CFLAGS initialization before including
tools/scripts/Makefile.include, so it is not affected by the extended
EXTRA_CFLAGS.

Append EXTRA_CFLAGS to CFLAGS only after including Makefile.include and
place it last so that the extra flags propagate properly and can
override the default options.

tools/scripts/Makefile.include already appends $(CLANG_CROSS_FLAGS) to
CFLAGS, the Makefile appends $(CLANG_CROSS_FLAGS) again, remove the
redundant append.

Signed-off-by: Leo Yan <leo.yan@arm.com>
Acked-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260602-tools_build_fix_zero_init_bpf_only-v2-4-c76e5250ea1c@arm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>