Add notpossible keyword

This is the same as `unreachable` except it always traps violations,
even if we're not running in MODE=dbg. This is useful for impossible
conditions relating to system calls. It avoids terrifying bugs where
control falls through to an unrelated function.

libc/integral/c.inc

@@ -591,6 +591,12 @@ typedef struct {
 #endif
 #endif
 
+#define notpossible \
+  do {              \
+    asm("hlt");     \
+    unreachable;    \
+  } while (0)
+
 #define donothing \
   do {            \
   } while (0)