Fill 'struct fpreg' & 'struct xmmregs' in

The 'struct fpreg' data type used by PT_GETFPREGS, and 'struct xmmregs'
used by PT_GETXMMREGS are currently opaque.  Define them to contain
correct data structs instead, the same way they are defined for amd64.
For 'struct fpreg' this means 'struct save87', and for 'struct xmmregs'
this means 'struct fxsave'.  This makes it more transparent for
consumers how the data is formatted, and allows using it without need
for explicit casts.

Reviewed by <kamil>.

(mgorny)