systemc: Add a non-standard default writer policy on sc_buffer.

The standard says there's not supposed to be a default writer policy
for the sc_buffer template class, but in the Accellera implementation
there is, and the regression tests depend on it to compile.

Change-Id: I31d17617441224e86c56c54e45364be8f4f45b00
Reviewed-on: https://gem5-review.googlesource.com/10957
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/systemc/ext/channel/sc_buffer.hh

diff --git a/src/systemc/ext/channel/sc_buffer.hh b/src/systemc/ext/channel/sc_buffer.hh
index 153b60b..2208160 100644
--- a/src/systemc/ext/channel/sc_buffer.hh
+++ b/src/systemc/ext/channel/sc_buffer.hh
@@ -37,7 +37,9 @@
 namespace sc_core
 {
 
-template <class T, sc_writer_policy WRITER_POLICY>
+// Having a default value for the WRITER_POLICY parameter is non-standard, but
+// matches the Accellera implementation to enable the regression tests.
+template <class T, sc_writer_policy WRITER_POLICY=SC_ONE_WRITER>
 class sc_buffer : public sc_signal<T, WRITER_POLICY>
 {
   public: