Enable POSIX threads for uClinux targets
Reported upstream:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71721

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

diff -Nur gcc-5.4.0.orig/gcc/config.gcc gcc-5.4.0/gcc/config.gcc
--- gcc-5.4.0.orig/gcc/config.gcc	2015-09-10 16:17:53.000000000 +0200
+++ gcc-5.4.0/gcc/config.gcc	2016-07-01 21:26:02.772958441 +0200
@@ -808,6 +808,9 @@
 *-*-uclinux*)
   extra_options="$extra_options gnu-user.opt"
   use_gcc_stdint=wrap
+  case ${enable_threads} in
+    "" | yes | posix) thread_file='posix' ;;
+  esac
   tm_defines="$tm_defines DEFAULT_LIBC=LIBC_UCLIBC SINGLE_LIBC"
   ;;
 *-*-rdos*)