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

tools/bpf/resolve_btfids/Makefile

@@ -20,6 +20,8 @@ LD       = $(HOSTLD)
 ARCH     = $(HOSTARCH)
 RM      ?= rm
 CROSS_COMPILE =
+CFLAGS  := $(KBUILD_HOSTCFLAGS)
+LDFLAGS := $(KBUILD_HOSTLDFLAGS)
 
 OUTPUT ?= $(srctree)/tools/bpf/resolve_btfids/
 
@@ -47,10 +49,10 @@ $(SUBCMDOBJ): fixdep FORCE | $(OUTPUT)/libsubcmd
 
 $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OUT)
 	$(Q)$(MAKE) $(submake_extras) -C $(LIBBPF_SRC) OUTPUT=$(LIBBPF_OUT)    \
-		    DESTDIR=$(LIBBPF_DESTDIR) prefix=			       \
+		    DESTDIR=$(LIBBPF_DESTDIR) prefix= EXTRA_CFLAGS="$(CFLAGS)" \
 		    $(abspath $@) install_headers
 
-CFLAGS := -g \
+CFLAGS += -g \
           -I$(srctree)/tools/include \
           -I$(srctree)/tools/include/uapi \
           -I$(LIBBPF_INCLUDE) \