Replace assert(0) with abort() or cpu_abort()

When building with -DNDEBUG, assert(0) will not stop execution
so it must not be used for abnormal termination.

Use cpu_abort() when in CPU context, abort() otherwise.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/target-sh4/README.sh4 b/target-sh4/README.sh4
index a92b6f3..e578830 100644
--- a/target-sh4/README.sh4
+++ b/target-sh4/README.sh4
@@ -6,7 +6,7 @@
 file describes the current state of implementation.
 
 Most places requiring attention and/or modification can be detected by
-looking for "XXXXX" or "assert (0)".
+looking for "XXXXX" or "abort()".
 
 The sh4 core is located in target-sh4/*, while the 7750 peripheral
 features (IO ports for example) are located in hw/sh7750.[ch]. The