disable split-stack for non-thread builds

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>

diff -Nur gcc-5.3.0.orig/libgcc/config/t-stack gcc-5.3.0/libgcc/config/t-stack
--- gcc-5.3.0.orig/libgcc/config/t-stack	2010-10-01 21:31:49.000000000 +0200
+++ gcc-5.3.0/libgcc/config/t-stack	2016-03-07 03:25:32.000000000 +0100
@@ -1,4 +1,6 @@
 # Makefile fragment to provide generic support for -fsplit-stack.
 # This should be used in config.host for any host which supports
 # -fsplit-stack.
+ifeq ($(enable_threads),yes)
 LIB2ADD_ST += $(srcdir)/generic-morestack.c $(srcdir)/generic-morestack-thread.c
+endif