kbuild: Fix 'no symbols' warning when CONFIG_TRIM_UNUSD_KSYMS=y

When CONFIG_TRIM_UNUSED_KSYMS is enabled, I see some warnings like this:

  nm: arch/x86/entry/vdso/vdso32/note.o: no symbols

$NM (both GNU nm and llvm-nm) warns when no symbol is found in the
object. Suppress the stderr.

Fangrui Song mentioned binutils>=2.37 `nm -q` can be used to suppress
"no symbols" [1], and llvm-nm>=13.0.0 supports -q as well.

We cannot use it for now, but note it as a TODO.

[1]: https://sourceware.org/bugzilla/show_bug.cgi?id=27408

Fixes: bbda5ec6 ("kbuild: simplify dependency generation for CONFIG_TRIM_UNUSED_KSYMS")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>

scripts/gen_ksymdeps.sh

@@ -4,7 +4,13 @@
 set -e
 
 # List of exported symbols
-ksyms=$($NM $1 | sed -n 's/.*__ksym_marker_\(.*\)/\1/p' | tr A-Z a-z)
+#
+# If the object has no symbol, $NM warns 'no symbols'.
+# Suppress the stderr.
+# TODO:
+#   Use -q instead of 2>/dev/null when we upgrade the minimum version of
+#   binutils to 2.37, llvm to 13.0.0.
+ksyms=$($NM $1 2>/dev/null | sed -n 's/.*__ksym_marker_\(.*\)/\1/p' | tr A-Z a-z)
 
 if [ -z "$ksyms" ]; then
 	exit 0