Merge branch 'tools-build-bpf-append-extra_cflags-and-host_extracflags'

Leo Yan says:

====================
tools build: bpf: Append EXTRA_CFLAGS and HOST_EXTRACFLAGS

Append EXTRA_CFLAGS and HOST_EXTRACFLAGS to the BPF build.

This mitigates an issue introduced in GCC 15, where a {0} initializer
does not guarantee zeroing the entire union [1].

The common changes under tools to support EXTRA_CFLAGS and
HOST_EXTRACFLAGS are sent separately [2].  As suggested, BPF patches
would be picked up via the bpf tree, so this series only includes BPF
related changes.

Verification on bpf-ci (with tools changes [2]:

  https://github.com/kernel-patches/bpf/actions/runs/26815163486

[1] https://gcc.gnu.org/gcc-15/changes.html
[2] https://lore.kernel.org/all/20260602-tools_build_fix_zero_init-v7-0-631baf679fe7@arm.com/

Signed-off-by: Leo Yan <leo.yan@arm.com>
---
Changes in v2:
- Used strscpy() instead in patch 06 (Ihor).
- Added prefix "bpf-next" in subject (Alexei).
- Added patch 01 to pass host cflags to bootstrap libbpf.
- Added patch 08 to avoid static LLVM linking for cross build.
- Link to v1: https://lore.kernel.org/r/20260323-tools_build_fix_zero_init_bpf_only-v1-0-d1cfad2f4cd1@arm.com
====================

Link: https://patch.msgid.link/20260602-tools_build_fix_zero_init_bpf_only-v2-0-c76e5250ea1c@arm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>