Restore the ABI of 'struct fpreg'.

[jhb]

Summary

Restore the ABI of 'struct fpreg' on powerpc.

The PT_{GET,SET}FPREGS requests use 'struct fpreg' and the NT_FPREGSET
core note stores a copy of 'struct fpreg'. As with x86 and the floating
point state there compared to the extended state in XSAVE, struct fpreg
on powerpc now only holds the 'base' FP state, and setting it via
PT_SETFPREGS leaves the extended vector state in a thread unchanged.

Test Plan

• Run a test program using floating point under a new kernel. Verify that gdb can read/write float registers correctly.
• Verify that the new kernel writes out the correct size core dumps and that gdb no longer complains about '.reg2' having an unexpected size.

[jhibbits]

Looks fine to me, as long as it passes your tests.