kbuild: avoid split lines in .mod files

"xargs echo" is not a safe way to remove line breaks because the input
may exceed the command line limit and xargs may break it up into
multiple invocations of echo. This should never happen because
scripts/gen_autoksyms.sh expects all undefined symbols are placed in
the second line of .mod files.

One possible way is to replace "xargs echo" with
"sed ':x;N;$!bx;s/\n/ /g'" or something, but I rewrote the code by
using awk because it is more readable.

This issue was reported by Sami Tolvanen; in his Clang LTO patch set,
$(multi-used-m) is no longer an ELF object, but a thin archive that
contains LLVM bitcode files. llvm-nm prints out symbols for each
archive member separately, which results a lot of dupications, in some
places, beyond the system-defined limit.

This problem must be fixed irrespective of LTO, and we must ensure
zero possibility of having this issue.

Link: https://lkml.org/lkml/2020/12/1/1658Reported-by: NSami Tolvanen <samitolvanen@google.com>
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Reviewed-by: NSami Tolvanen <samitolvanen@google.com>

scripts/Makefile.build

@@ -252,6 +252,9 @@ objtool_dep = $(objtool_obj)					\
 ifdef CONFIG_TRIM_UNUSED_KSYMS
 cmd_gen_ksymdeps = \
 	$(CONFIG_SHELL) $(srctree)/scripts/gen_ksymdeps.sh $@ >> $(dot-target).cmd
+
+# List module undefined symbols
+undefined_syms = $(NM) $< | $(AWK) '$$1 == "U" { printf("%s%s", x++ ? " " : "", $$2) }';
 endif
 
 define rule_cc_o_c
@@ -271,13 +274,6 @@ define rule_as_o_S
 	$(call cmd,modversions_S)
 endef
 
-# List module undefined symbols (or empty line if not enabled)
-ifdef CONFIG_TRIM_UNUSED_KSYMS
-cmd_undef_syms = $(NM) $< | sed -n 's/^  *U //p' | xargs echo
-else
-cmd_undef_syms = echo
-endif
-
 # Built-in and composite module parts
 $(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
 	$(call if_changed_rule,cc_o_c)
@@ -285,7 +281,7 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
 
 cmd_mod = { \
 	echo $(if $($*-objs)$($*-y)$($*-m), $(addprefix $(obj)/, $($*-objs) $($*-y) $($*-m)), $(@:.mod=.o)); \
-	$(cmd_undef_syms); \
+	$(undefined_syms) echo; \
 	} > $@
 
 $(obj)/%.mod: $(obj)/%.o FORCE