sim: or1k: build with -Werror

The only warnings left in this port are a few maybe-uninitialized,
but we don't abort the build for them, so turn on -Werror everywhere.

sim/or1k/Makefile.in

diff --git a/sim/or1k/Makefile.in b/sim/or1k/Makefile.in
index 8f447fd..c6bd03b 100644
--- a/sim/or1k/Makefile.in
+++ b/sim/or1k/Makefile.in
@@ -55,9 +55,6 @@
 
 SIM_EXTRA_CLEAN = or1k-clean
 
-# Code doesn't build cleanly yet.
-SIM_WERROR_CFLAGS =
-
 ## COMMON_POST_CONFIG_FRAG
 
 arch = or1k