libc: fix linker script generation for libubacktrace.so

Fix libc.so linker script generation to add libubacktrace AS_NEEDED entry
using MULTILIB_DIR instead of hard-coded "lib".

Signed-off-by: Filippo Arcidiacono <filippo.arcidiacono@st.com>
Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>

Makefile.in

@@ -333,8 +333,8 @@ else
 endif
 ifeq ($(UCLIBC_HAS_BACKTRACE),y)
 # Add the AS_NEEDED entry for libubacktrace.so
-	if [ -f $(top_builddir)lib/libc.so -a -f $(PREFIX)$(RUNTIME_PREFIX)lib/$(SHARED_MAJORNAME) ] ; then \
-		echo "GROUP ( $(UBACKTRACE_ASNEEDED) )" >> $(PREFIX)$(DEVEL_PREFIX)lib/libc.so; \
+	if [ -f $(top_builddir)lib/libc.so -a -f $(PREFIX)$(RUNTIME_PREFIX)$(MULTILIB_DIR)/$(SHARED_MAJORNAME) ] ; then \
+		echo "GROUP ( $(UBACKTRACE_ASNEEDED) )" >> $(PREFIX)$(DEVEL_PREFIX)$(MULTILIB_DIR)/libc.so; \
 	fi
 endif
 ifeq ($(UCLIBC_HAS_THREADS),y)