Split off record layouting

Such struct decl:

  struct S { char a; int i;} __attribute__((packed));

should be accepted and cause S to be five bytes long (i.e.
the packed attribute should matter).  So we can't layout
the members during parsing already.  Split off the offset
and alignment calculation for this.

tests/tcctest.c

@@ -3539,7 +3539,6 @@ typedef struct __attribute__((__packed__)) {
     int c;
 } Spacked2;
 Spacked2 spacked2;
-#ifdef BROKEN
 /* This doesn't work for now.  Requires adjusting field offsets/sizes
    after parsing the struct members.  */
 typedef struct Spacked3_s {
@@ -3548,14 +3547,11 @@ typedef struct Spacked3_s {
     int c;
 } __attribute__((__packed__)) Spacked3;
 Spacked3 spacked3;
-#endif
 void attrib_test(void)
 {
   printf("attr: %d %d %d %d\n", sizeof(struct Spacked),
 	 sizeof(spacked), sizeof(Spacked2), sizeof(spacked2));
-#ifdef BROKEN
   printf("attr: %d %d\n", sizeof(Spacked3), sizeof(spacked3));
-#endif
 }
 extern __attribute__((__unused__)) char * __attribute__((__unused__)) *
 strange_attrib_placement (void);