Correct definition of bits per word, as reported in #196. (#217)

LibOS/shim/include/shim_signal.h

@@ -13,7 +13,7 @@ struct shim_signal_handle {
     struct __kernel_sigaction * action;
 };
 
-# define BITS_PER_WORD sizeof(unsigned long)
+# define BITS_PER_WORD (8 * sizeof(unsigned long))
 /* The standard def of this macro is dumb */
 #undef _SIGSET_NWORDS
 # define _SIGSET_NWORDS (NUM_SIGS / BITS_PER_WORD)