Fix regarding issues on Windows ARM64

I have tried to get FPC running on Windows for ARM64.
And I must say: everything was there already !
Good work FPC devs !!

However, all binaries did crash on exit.
After a lot of debugging, I found two issues that seem to solve this.

1:
TTlsDirectory={packed} record <--- do not pack on ARM64

2:

procedure SysInitThreadvar(var offset : dword;size : dword);
  begin
    offset:=threadvarblocksize;
    {$ifdef CPUAARCH64} // Specific to ARM64
    size := (size + 7) and not dword(7); <---- align 8 on ARM64
    {$else ifdef CPUARM} // ARM32
    size := (size + 3) and not dword(3);
    {$endif}        inc(threadvarblocksize,size);
  end;

For me, these changes create working binaries on ARM64 !