Allow [U]LongInt bitfields on 64bit

Since 64bit Integer bitfields are allowed on 64bit already, it should
be possible to allow LongInt too, on 64bit at least. Unfortunately, the
test case shows that 64bit bitfields aren't working properly on 64bit yet,
no matter whether using Integer or LongInt types.

Allowing 64bit bitfields on 32bit is a different story though, because
currently the bitfield access code always uses UInteger (thus can't
handle bitfields > 32bit on 32bit).

This helps supporting the new 64bit Windows API headers which contain
such bitfields with LongInt type (but in the 64bit part of the API only).