tools/resolve_btfids: Fix setting HOSTCFLAGS

Bugzilla: https://bugzilla.redhat.com/2178930
Upstream status: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git

commit edd75c8
Author: Viktor Malik <vmalik@redhat.com>
Date:   Tue May 30 14:33:52 2023 +0200

    tools/resolve_btfids: Fix setting HOSTCFLAGS

    Building BPF selftests with custom HOSTCFLAGS yields an error:

        # make HOSTCFLAGS="-O2"
        [...]
          HOSTCC  ./tools/testing/selftests/bpf/tools/build/resolve_btfids/main.o
        main.c:73:10: fatal error: linux/rbtree.h: No such file or directory
           73 | #include <linux/rbtree.h>
              |          ^~~~~~~~~~~~~~~~

    The reason is that tools/bpf/resolve_btfids/Makefile passes header
    include paths by extending HOSTCFLAGS which is overridden by setting
    HOSTCFLAGS in the make command (because of Makefile rules [1]).

    This patch fixes the above problem by passing the include paths via
    `HOSTCFLAGS_resolve_btfids` which is used by tools/build/Build.include
    and can be combined with overridding HOSTCFLAGS.

    [1] https://www.gnu.org/software/make/manual/html_node/Overriding.html

    Fixes: 56a2df7 ("tools/resolve_btfids: Compile resolve_btfids as host program")
    Signed-off-by: Viktor Malik <vmalik@redhat.com>
    Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
    Acked-by: Jiri Olsa <jolsa@kernel.org>
    Link: https://lore.kernel.org/bpf/20230530123352.1308488-1-vmalik@redhat.com

Signed-off-by: Viktor Malik <vmalik@redhat.com>

Author: viktormalik

tools/bpf/resolve_btfids/Makefile

@@ -67,7 +67,7 @@ $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OU
 LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
 LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
 
-HOSTCFLAGS += -g \
+HOSTCFLAGS_resolve_btfids += -g \
           -I$(srctree)/tools/include \
           -I$(srctree)/tools/include/uapi \
           -I$(LIBBPF_INCLUDE) \
@@ -76,7 +76,7 @@ HOSTCFLAGS += -g \
 
 LIBS = $(LIBELF_LIBS) -lz
 
-export srctree OUTPUT HOSTCFLAGS Q HOSTCC HOSTLD HOSTAR
+export srctree OUTPUT HOSTCFLAGS_resolve_btfids Q HOSTCC HOSTLD HOSTAR
 include $(srctree)/tools/build/Makefile.include
 
 $(BINARY_IN): fixdep FORCE prepare | $(OUTPUT)