bpftool: Strip all -Wformat* flags from bootstrap libbpf build

Commit 9080b97689 ("bpftool: Pass host flags to bootstrap libbpf")
started building the bootstrap libbpf with HOST_CFLAGS, stripping the
warning options that are unsuitable for that build by filtering out
-W -Wall -Wextra -Wformat -Wformat-signedness.

HOST_CFLAGS inherits EXTRA_WARNINGS, which includes -Wformat-security
and -Wformat-y2k. The filter drops -Wall and -Wformat (the latter being
what actually enables -Wformat), but leaves those two -Wformat-* children
in LIBBPF_BOOTSTRAP_CFLAGS. Building the bootstrap libbpf with it then
warns:

  cc1: warning: '-Wformat-y2k' ignored without '-Wformat'
  cc1: warning: '-Wformat-security' ignored without '-Wformat'

The warning is easy to miss in an in-tree build: tools/lib/bpf/Makefile
re-adds -Wall via "override CFLAGS += -Wall", which re-enables -Wformat
for the libbpf objects, so only libbpf's feature-detection probe (which
uses the passed CFLAGS verbatim) leaks the two warnings. The standalone
libbpf Makefile (github.com/libbpf/libbpf, used by the bpftool mirror)
instead uses "CFLAGS ?= ... -Wall", which the passed-in CFLAGS overrides,
so -Wall is never re-added and every bootstrap object warns.

Use a -Wformat% wildcard in the filter-out so the orphaned children are
removed together with the parent.

Fixes: 9080b97689 ("bpftool: Pass host flags to bootstrap libbpf")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/bpf/20260630205418.3483969-1-andrii@kernel.org

tools/bpf/bpftool/Makefile

@@ -99,7 +99,7 @@ endif
 HOST_LDFLAGS := $(LDFLAGS)
 
 # Remove warnings for libbpf bootstrap build
-LIBBPF_BOOTSTRAP_CFLAGS := $(filter-out -W -Wall -Wextra -Wformat -Wformat-signedness,$(HOST_CFLAGS))
+LIBBPF_BOOTSTRAP_CFLAGS := $(filter-out -W -Wall -Wextra -Wformat%,$(HOST_CFLAGS))
 
 INSTALL ?= install
 RM ?= rm -f