bfin: fix missing REENTRANT cpp flag

toolchain/gcc/patches/6.2.0/bfin-define-REENTRANT.patch

@@ -0,0 +1,13 @@
+diff -Nur gcc-6.2.0.orig/gcc/config/bfin/linux.h gcc-6.2.0/gcc/config/bfin/linux.h
+--- gcc-6.2.0.orig/gcc/config/bfin/linux.h	2016-01-04 15:30:50.000000000 +0100
++++ gcc-6.2.0/gcc/config/bfin/linux.h	2016-09-30 20:48:17.446636819 +0200
+@@ -38,6 +38,9 @@
+   "%{static:--start-group} %{mfast-fp:-lbffastfp} %G %L %{static:--end-group} \
+    %{!static:%{mfast-fp:-lbffastfp} %G}"
+ 
++#undef CPP_SPEC
++#define CPP_SPEC "%{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}"
++
+ #undef LINK_SPEC
+ #define LINK_SPEC "\
+   %{mfdpic: -m elf32bfinfd -z text} %{shared} %{pie} \