ffi: bitfield handling when storage cells are packed.

In gcc, bitfields can be declared with overridden alignment;
for instance when the entire structure is packed.
We must modify the algorithm to make this work.

Firstly, we must must pretend that we are filling from the start
of the containing alignment unit, not size based unit. For
instance, if the cell is an 8 byte type, but the alignment is 1,
then we are just filling the current byte, not the current 8 byte
multiple. However, the number of bits left (room) is still
calculated from the size: so if the byte has been filled with 3 bits,
we have 4 left in that byte, and then 7 more bytes, and so 61 bits.

Secondly, the case of the zero-bit field is special: it does
perform an alignment calculation in terms of multiples of the
size, ignoring the declared alignment of the field.

Thirdly, when the bits do not fit into the existing room, the
offset does not move by the size of the bitfield, but by the
alignment unit. For instance, if we have 63 bits left in an 8
byte field that has alignment 1, and we want to place 64 bits
into another 8 byte field, then we will just move to the next
byte, not to the next 64 bit cell. Starting at the next byte,
we pretend we have a fres 64 bit cell, albeit misaligned.

* ffi.c (make_ffi_type_struct): Make the above adjustments to
the algorithm. Some common values size, alignment, mask)
are declared in a wider scope and adjustments made.

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