Make TOK_alloca available for x86-64

TOK_alloca is now available on x86-64 so make put definition of
TOK_alloca outside the BCHECK conditional macro definition but test if
arch is i386 or x86-64. This makes C99 VLA works (understand compile) on
x86-64.

tcctok.h

@@ -237,6 +237,8 @@
      DEF(TOK_memmove, "memmove")
      DEF(TOK_strlen, "strlen")
      DEF(TOK_strcpy, "strcpy")
+#endif
+#if defined __i386__ || defined __x86_64__
      DEF(TOK_alloca, "alloca")
 #endif