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>

Index: b/gcc/config.gcc
===================================================================
--- a/gcc/config.gcc
+++ b/gcc/config.gcc
@@ -810,6 +810,9 @@
 *-*-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*)