FROMGIT: tools/resolve_btfids: Build with host flags

resolve_btfids is built using $(HOSTCC) and $(HOSTLD) but does not
pick up the corresponding flags. As a result, host-specific settings
(such as a sysroot specified via HOSTCFLAGS=--sysroot=..., or a linker
specified via HOSTLDFLAGS=-fuse-ld=...) will not be respected.

Fix this by setting CFLAGS to KBUILD_HOSTCFLAGS and LDFLAGS to
KBUILD_HOSTLDFLAGS.

Also pass the cflags through to libbpf via EXTRA_CFLAGS to ensure that
the host libbpf is built with flags consistent with resolve_btfids.

Signed-off-by: Connor O'Brien <connoro@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20220112002503.115968-1-connoro@google.com
(cherry picked from commit 0e3a1c90
 git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git

 master)
Bug: 203823368
Test: build with CONFIG_DEBUG_INFO_BTF=y
Signed-off-by: Connor O'Brien <connoro@google.com>
Change-Id: I09ee10b29b57933653eb1cdd4249bac2d9cebf22