sim: split sim-signal.h include out

The sim-basics.h is too big and includes too many things.  This leads
to some arch's sim-main.h having circular loop issues with defs, and
makes it hard to separate out common objects from arch-specific defs.
By splitting up sim-basics.h and killing off sim-main.h, it'll make
it easier to separate out the two.

sim/lm32/traps.c

@@ -25,6 +25,7 @@
 #define WANT_CPU_LM32BF
 
 #include "sim-main.h"
+#include "sim-signal.h"
 #include "sim-syscall.h"
 #include "lm32-sim.h"
 #include "targ-vals.h"