Separate the alignment of a field from the alignment of the
type of the field. Use the field alignment to compute the
size of a structure.
This may help 8g but is mainly for gccgo. gccgo maintains the
standard C/C++ ABI for structure field alignment. For the
i386, this requires that a float64 field in a struct be
aligned on a 32-bit boundary, although for efficiency a
variable of type float64 or []float64 should be aligned on a
64-bit boundary.
I also removed the unused size field from structField.