m68k: access to unaligned data in packed records

When accessing unaligned data in packed records, wrong code is generated for -Cp68000.

The following example:

type
  THeader = packed record
    Signature:  char;     { signature byte                        }
    Nr_chars:   smallint;  { number of characters in file          }
  end;


var t: Theader;

begin
	t.nr_chars := swap(t.nr_chars);
end.

generates:

main:
[0000012c] 4eb9 0000 1368            jsr        FPC_INITIALIZEUNITS
[00000132] 41f9 0000 3811            lea.l      $00003811,a0
[00000138] 3010                      move.w     (a0),d0
[0000013a] c0bc 0000 ffff            and.l      #$0000FFFF,d0
[00000140] 2200                      move.l     d0,d1
[00000142] e089                      lsr.l      #8,d1
[00000144] e188                      lsl.l      #8,d0
[00000146] 8280                      or.l       d0,d1
[00000148] c2bc 0000 ffff            and.l      #$0000FFFF,d1
[0000014e] 41f9 0000 3812            lea.l      $00003812,a0
[00000154] 1081                      move.b     d1,(a0)
[00000156] e049                      lsr.w      #8,d1
[00000158] 1101                      move.b     d1,-(a0)
[0000015a] 4eb9 0000 1548            jsr        FPC_DO_EXIT

When reading the nr_chars member, a word access to an odd address is done. When writing it back, it is correctly done using two single-byte accesses.

Another note: the code was compiled with -O2, but looks really inefficient.