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.

R=r
DELTA=117  (75 added, 2 deleted, 40 changed)
OCL=26842
CL=26936