bpftool: Add missing quotes to libbpf bootstrap submake vars

When passing compiler variables like CC=$(HOSTCC) to a submake
we must ensure the variable is quoted in order to handle cases
where $(HOSTCC) may be multiple binaries.

For example when using ccache $HOSTCC may be:
"/usr/bin/ccache /usr/bin/gcc"

If we pass CC without quotes like CC=$(HOSTCC) only the first
"/usr/bin/ccache" part will be assigned to the CC variable which
will cause an error due to dropping the "/usr/bin/gcc" part of
the variable in the submake invocation.

This fixes errors such as:
/usr/bin/ccache: invalid option -- 'd'
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20230110014504.3120711-1-james.hilliard1@gmail.com

tools/bpf/bpftool/Makefile

@@ -53,7 +53,7 @@ $(LIBBPF_INTERNAL_HDRS): $(LIBBPF_HDRS_DIR)/%.h: $(BPF_DIR)/%.h | $(LIBBPF_HDRS_
 $(LIBBPF_BOOTSTRAP): $(wildcard $(BPF_DIR)/*.[ch] $(BPF_DIR)/Makefile) | $(LIBBPF_BOOTSTRAP_OUTPUT)
 	$(Q)$(MAKE) -C $(BPF_DIR) OUTPUT=$(LIBBPF_BOOTSTRAP_OUTPUT) \
 		DESTDIR=$(LIBBPF_BOOTSTRAP_DESTDIR:/=) prefix= \
-		ARCH= CROSS_COMPILE= CC=$(HOSTCC) LD=$(HOSTLD) AR=$(HOSTAR) $@ install_headers
+		ARCH= CROSS_COMPILE= CC="$(HOSTCC)" LD="$(HOSTLD)" AR="$(HOSTAR)" $@ install_headers
 
 $(LIBBPF_BOOTSTRAP_INTERNAL_HDRS): $(LIBBPF_BOOTSTRAP_HDRS_DIR)/%.h: $(BPF_DIR)/%.h | $(LIBBPF_BOOTSTRAP_HDRS_DIR)
 	$(call QUIET_INSTALL, $@)