Add tests that trap, and fix handling of SIGFPE on x86.

filetests/trap_divbyzero.wat

@@ -0,0 +1,11 @@
+(module
+  (func $foo (result i32)
+    i32.const 1
+    i32.const 0
+    i32.div_s
+  )
+  (func $main
+    (drop (call $foo))
+  )
+  (start $main)
+)

filetests/trap_stackoverflow.wat

@@ -0,0 +1,9 @@
+(module
+  (func $foo
+    (call $main)
+  )
+  (func $main
+    (call $foo)
+  )
+  (start $main)
+)

filetests/trap_unreachable.wat

@@ -0,0 +1,9 @@
+(module
+  (func $foo
+    (unreachable)
+  )
+  (func $main
+    (call $foo)
+  )
+  (start $main)
+)