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

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>